APPARATUS FOR ADJUSTING TRIM OF A MARINE HULL Technical Field The present invention relates to marine hulls, and particularly relates to apparatus for adjusting the trim of a marine hull. Background of the Invention Power driven planing marine hulls typically require a large amount of power and corresponding high speed to bring the hull onto plane and continued high throttle settings and speed to maintain the plane. Planing hulls typically ride with the bow elevated high above the water whilst getting on the plane. The bow may also sit relatively high whilst on the plane, although at a lower position to that during transition getting on the plane. Travelling with the bow high results in some lateral hull instability, inhibiting the ability to carry passengers or cargo comfortably, and also inhibits driver vision. Planing hulls that are heavily loaded often have difficulty in obtaining, or are unable to obtain, a planing altitude. Many semi-planing and displacement type hulls also experience difficulty in carrying large loads, with the stern of the hull riding low in the water. To alleviate some of the problems listed above, trim tabs are known to be mounted behind the transom, or under the aft sections, of a planing hull and adjusted mechanically in an attempt to better trim the hull to a more horizontal orientation. Trim tabs operate by deflecting water downward after having left the running surface behind the transom. Such trim tabs are, however, vulnerable to damage and are relatively inefficient. The use of retractable blades mounted on the transom of a planing hull has also been proposed to adjust the trim of the marine hull by generating lift on the rear portions of the hull immediately forward of the displaceable blade. The retractable blades, when deployed into the water flowpath beneath the transom, act to slow/dam the water flow in front of the blade thereby increasing the water pressure acting on the hull forward of the blade. The increased lift generated on the rear portions of the hull act to lift the aft end of the hull, assisting planing and semi-planing hulls getting onto and maintaining the plane. Currently proposed blade systems, however, have relatively complex systems located adjacent or below the water line which may be subjected to ingress of water and subsequent degradation, particularly by corrosion damage. Object of the Invention It is the object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages. Summary of the Invention There is disclosed herein apparatus for adjusting the trim of a marine hull having a transom and an underside running surface terminating in a trailing edge at said transom, said apparatus comprising: a blade having an upper edge and a lower edge; an expandable chamber located above said blade, said expandable chamber being defined by a chamber wall, at least part of said chamber wall being formed of an elastically deformable material, said chamber wall including a lower chamber wall portion fixed to said blade upper edge; a pressure supply communicating with said expandable chamber for expanding said expandable chamber from a non-pressurised state to an expanded, pressurised state by elastically deforming said elastically deformable material; wherein said blade is adapted to be slidably mounted with respect to said marine , hull, with said blade extending laterally along or adjacent to said running surface, such that expansion of said chamber from said non-pressurised state to said expanded, pressurised state slidingly displaces said blade from a retracted position above said running surface to an extended position with said blade lower edge protruding below said running surface. The blade may be adapted to be slidably mounted withrespect to said marine hull with said blade extending laterally along or adjacent to said running surface trailing edge. In one form, said chamber wall is in the form of a bladder formed of said elastically deformable material. The blade may be elastically deformable. The blade may be formed of an elastomeric material. The elastomeric material may be neoprene or rubber. The blade may be integrally formed with said bladder. The apparatus may further comprise a housing adapted to be secured to said transom, said chamber and at least said blade upper edge being located within said housing. An upper portion of said bladder will typically be fixed in relation to said housing. In one form, said housing has an open end enabling said bladder and said blade to be extracted from said housing, said housing further having a cap for closing said open end. In another form, said chamber wall is defined by said lower chamber wall portion and a rigid upper chamber wall portion fixed in relation to said marine hull, said lower chamber wall portion being formed of said elastically deformable material. The rigid upper chamber wall portion may be defined by said housing. In an alternate form, said chamber may be located within a cavity provided in said marine hull and opening onto said running surface adjacent said running surface trailing edge. Said chamber wall may be defined by said lower chamber wall and a rigid upper chamber wall portion of said cavity, said lower chamber wall portion being formed of an elastically deformable material. There is further disclosed herein a marine vessel comprising: a marine hull having a transom and an underside running surface terminating in a trailing edge at said transom; and apparatus for adjusting the trim of said marine hull as defined above, said blade of said apparatus being slidably mounted with respect to said marine hull with said blade extending laterally along or adjacent to said running surface. Brief Description of the Drawings Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: Figure 1 is a side elevation view of a marine hull incorporating a first apparatus for adjusting trim of the marine hull. Figure 2 is a cross-sectional side elevation view of the apparatus of Figure 1. Figure 3 is a rear, perspective view of the apparatus of Figure 1. Figure 4 is a cross-sectional side elevation view of a second apparatus for adjusting trim of a marine hull with the blade in a retracted position. Figure 5 is a cross-sectional side elevation view of the apparatus of Figure 4 with the blade in an extended position. Figure 6 is a cross-sectional side elevation view of a third apparatus for adjusting trim of a marine hull. Figure 7 is a rear perspective view of the apparatus of Figure 6. Figure 8 is a cross-sectional side elevation view of a fourth apparatus for adjusting trim of a marine hull. Figure 9 is a cross-sectional side elevation view of a fifth apparatus for adjusting trim of a marine hull with the blade in a retracted position. Figure 10 is a cross-sectional side elevational view of the apparatus of Figure 9 with the blade in an extended position. Figure 11 is a cross-sectional side elevational view of a sixth apparatus for adjusting trim of a marine hull. Figure 12 is a cross-sectional side elevational view of a seventh apparatus for adjusting trim of a marine hull with the blade in a retracted position. Figure 13 is a side elevation view of the apparatus of Figure 12 with the blade in an extended position. Figure 14 is a side elevation view of an eighth apparatus for adjusting trim of a marine hull. Figure 15 is a cross-sectional side elevation view of a ninth apparatus for adjusting trim of a marine hull. Figure 16 is a cross-sectional side elevation view of a tenth apparatus for adjusting trim of a marine hull. Figure 17 is a schematic diagram of the pressure and control circuit of an apparatus for adjusting trim of a marine hull. Figure 18 is a schematic plan view of the pressure and control circuit of Figure 17. Figure 19 is a schematic diagram of an alternative pressure and control circuit. Figure 20 is a schematic diagram of another alternative pressure and control circuit. Detailed Description of the Preferred Embodiments Referring to Figure 1 of the accompanying drawings, a marine hull 1 of a marine vessel has a running surface 2 terminating in a trailing edge 3 forming the junction between the running surface 2 and the transom 4 of the hull. A first apparatus 10 for adjusting the trim of the marine hull 1, depicted in detail in Figures 2 and 3, includes a blade 11 adapted to be mounted to the marine hull 2, here by way of a housing 12. When the blade 11 and housing 12 are mounted on the marine hull 2, the blade 11 extends laterally along or adjacent to the running surface trailing edge 3. The blade 11 is displaceable between a retracted position above the running surface trailing edge 3, as depicted in Figure 2, and an extended position where the blade lower edge 1 Ia protrudes below the running surface trailing edge 3, as depicted with broken lines in Figure 2 (and, with less detail in Figure 1). When the blade 11 is in the extended position, water flowing past the running surface 2 is slowed by the leading face of the blade 11, increasing the water lift pressure acting on the running surface 2 in regions immediately forward of the blade 11, thereby generating increased lift acting on the rear portions of the marine hull 1. As a result, the transom 4 rides higher in the water W, with the bow 5 of the hull correspondingly riding lower above the water W. Stability, visibility and wakesize are thus improved. The increased lift lifts the marine hull 1 out of the water W more readily, so that a planing marine hull reaches the plane at a lower speed and with less power. The marine hull 1 is also able to stay on the plane with less power and less speed, although once speeds above planing speeds have been reached, the blades will typically be retracted. Increased cargo and passenger loads are also able to be carried while still maintaining the plane. Utilisation of such a blade 11 is also envisaged on semi-displacement and displacement type hulls to reduce the drag of such hulls. Such semi-displacement and displacement type hulls may regularly ride with a bow high and stern down trim, and the extra development of lift under the rear sections of the hull will improve the trim. The blade 11 may be elastically deformable, here being formed of an elastomeric material such as neoprene or rubber. Providing such an elastically deformable blade 11 reduces the possibility of damage to the blade 11 when in the extended position if it impacts any foreign objects passing underneath the marine hull 1. Arrangements are envisaged, however, where the blade 11 is rigid. The blade 11 is arranged to slidingly displace between the retracted and extended positions. In the extended position, the blade may protrude approximately 15 mm below the running surface trailing edge 3 for planing marine hulls up to about 10 m in length. Blade protruding lengths of the order of 40 mm may be utilised for larger hulls. However, the most appropriate blade protruding length will be more dependant on the specific hull design rather than the hull length, with poorly balanced hulls using deeper blade protruding depths. The person skilled in the art will be able to determine the most appropriate blade protruding length through trial and error. The blade 11 may have a thickness of the order of 10 to 15 mm. The blade is here arranged generally parallel to the transom 4, such that where the transom 4 is inclined at an obtuse angle to the running surface 2, the blade 11 leading face is inclined slightly forward in relation to an axis extending perpendicular to the running surface 2. The transom 4 will, however, often be inclined at a right angle or acute angle to the running surface 2. The blade 11 will be useful over a range of angles of inclination, but will typically be arranged either generally perpendicular to the running surface 2 or 5 inclined (fore or aft) within about 15 degrees of perpendicular. The inclination of the blade 11 leading face may be readily varied if so desired by mounting a wedge shaped base between the transom 4 and housing 12. This will be particularly appropriate if the transom 4 is inclined by more than 15 degrees from perpendicular to the running surface 2. io The blade 11 is displaced between the retracted and extended positions by virtue of an expandable chamber 13 located above the blade 11. In the arrangement of Figures 1 to 3, the chamber 13 is defined by a bladder 14, the lower wall 14a of which is fixed to the upper edge 1 Ib of the blade 11. The bladder 14 and blade 11 are integrally formed of the same elastomeric material, typically by moulding. A pressure supply 15 communicates I5 with the expandable chamber 13 via a conduit 24 for expanding the chamber 13 from a non-pressurised state, with the blade 11 in the retracted position, to an expanded, pressurised state with the blade 11 in the extended position. The pressure supply 15 will typically pressurise the chamber 13 with an hydraulic fluid, such as a water based liquid containing an anti-freeze coolant. The chamber 13 could alternatively be pressurised with 20 a gas. A pressure of the order of 25 to 100 psi (170 to 670 fcPa) will typically be appropriate, dependant on the size of the bladder 14 and thickness of the bladder walls. When the chamber 13 is in the expanded, pressurised state an electrically controlled valve will be closed to maintain pressure within the chamber 13. To de-pressurise the chamber 13, and thereby retract the blade 11, the valve be opened allowing fluid to return to the 25 pressure supply 15. The housing 12 is secured to the transom 4 of the marine hull 1, here by way of fasteners 16. The lower fasteners 16 extend through elongate slots l ie provided in the blade 11, allowing extension and retraction of the blade 11 without restriction from the fasteners 16. Bushings 16a are mounted in the lower fasteners 16 within the slots l ie. 30 The housing 12 is formed of two fibre reinforced resin moulded halves 17, 18, but may be formed of other suitable materials. The bladder 14 and blade 11 are housed between the two housing halves 17, 18 in a cavity 19 defined between the two housing halves 17, 18. A bulbous portion 20 is formed on the top of the bladder 14 and is fixedly held in a corresponding recess 21 communicating with the housing cavity 19. The bulbous portion 20 holds the upper portion of the bladder 14 in place. Accordingly, when the chamber 13 is pressurised, the upper portion of the bladder 14 remains generally fixed in place with the bladder 14 expanding downwardly along the housing cavity 19, displacing the bladder lower wall 14a and blade 11 in a downward direction. The blade lower edge 1 Ia is thus extended beneath the lower end of the housing 12 such that it protrudes beneath the running surface trailing edge 3. As best depicted in Figure 3, the housing cavity 19 may have an open lateral end 22 which enables the bladder 14 and blade 11 to be extracted from the housing 12 for replacement if damaged. A cap 23 is fastened onto the end of the housing 12 so as to close the open end 22. Two housings 12, each carrying a bladder 14 and blade 11, will typically be mounted on the marine hull 1, with one housing being located on each lateral side of the transom 4. The open end 22 of each housing 12 will typically be located at the outboard end to facilitate easy removal of the bladder 14 and blade 11. The bladder 14 and blade 11 can alternatively be accessed by removing the outer half 18 of the housing 12. The lower portion 19a of the housing cavity 19, through which the blade 11 slides, is generally not sealed around the blade 11. Water is thus able to pass into the main portion of the cavity 19 adjacent the bladder lower wall 14a. Any resultant water pressure acting against the exterior face of the bladder lower wall 14a will assist in returning the blade 11 to the retracted position when the pressure in the bladder 14 is relieved. The primary force retracting the blade 11 will, however, be provided by the elastic memory of the elastomeric bladder material that is effectively anchored by the bulbous portion 20. The use of the expandable bladder 14 to displace the blade 11 provides a very simple mechanism without moving parts that is robust and not subject to degradation from salt water ingress. When two blades 11 are mounted on the marine hull, one on either lateral side of the transom 4, the blades can be extended asymmetrically so as to adjust the lateral trim of the marine hull 1. Alternatively, a single blade 11 may extend across the transom 4. Figures 4 and 5 depict a second apparatus 110 for adjusting the trim of a marine hull 1, with the blade 111 in the retracted and extended positions respectively. Similarly to the first apparatus 10 of Figures 1 to 3, the second apparatus 110 includes an integrally formed elastomeric blade 111 and bladder 114 housed within the cavity 119 of a two part housing 112 fastened to the hull transom 4. The bladder chamber 113 communicates via a conduit 124 with a pressure supply 115 for expanding the bladder 114. The cavity 119 of the housing 112 is formed of three portions. The lower portion 119a of the cavity slidably houses the blade 111. The upper two portions 119b, 119c are defined generally as two overlapping cylinders, with the uppermost portion 119b being formed' as a larger cylinder than that of the middle portion 119c. The bladder 114 is formed such that, in the non- pressurised state as depicted in Figure 4, it has a generally cylindrical form conforming with that of the uppermost cavity portion 119b. Accordingly, when in the unpressurised state, the bladder 114 naturally rests in the uppermost cavity 119b, such that the blade 111 is retracted. When the bladder 114 is pressurised, the bladder lower wall 114a, is forced downwardly into the smaller middle portion 119c of the housing cavity 119, displacing the blade 111 downwardly to the extended position as depicted in Figure 5. Release of the pressure from the bladder 114 results in the bladder 114 returning to its undeformed state in the uppermost cavity 119b. Figures 6 and 7 depict a third apparatus 210 for adjusting the trim of a marine hull. This third apparatus 210 is similar to the second apparatus of Figures 4 and 5, except that the housing 212 is formed as a single piece moulding. The housing 212 may be formed of a material exhibiting a small amount of flexibility, such as a plastics material, to enable the housing 212 to be deformed so as to conform to the face of a contoured transom. The flexible housing 212 will also enable deformation of the housing 212 to enable the lower edge of the housing to conform to the running surface trailing edge 3, which will often be curved when viewed from the rear. To enable insertion and removal of the integral bladder 214 and blade 211, an open end 222 is provided on the housing 212 similar to that of the first apparatus shown in Figure 3. Again, a cap 223 is fastened to the open end 222 of the housing. The conduit 224 communicating the bladder chamber 213 with the pressure supply 215 may be routed through the cap 223. Figure 8 depicts a fourth apparatus 310 for adjusting the trim of a marine hull 1. Rather than using an elastomeric bladder to fully define the chamber 313, the expandable chamber 313 is defined by an elastically flexible membrane 314 and the upper half 317 of a two part housing 312. The membrane 314 defines the lower wall of the chamber 313 whilst the housing upper half 317 defines a rigid upper wall of the chamber 313. The elastic membrane 314 is integrally formed with the blade 311 of an elastomeric material to form a T shaped member. The opposing free end portions of the flexible membrane 314 are fastened between the upper and lower halves 317, 318 of the housing 312 by way of fasteners 325. The housing 312 is fastened to the transom 4 by any suitable means. The chamber 313 communicates with a pressure supply 315 byway of a conduit 324 passing through the housing upper half 317. When the chamber 313 is unpressurised, the elastic membrane 314 extends across the upper wall of the chamber 313, as depicted in broken lines in Figure 8. The blade 311 remains retracted in this state. When the chamber 313 is pressurised, the elastic membrane 314 is elastically stretched and deflected downwardly across the recessed cavity 319 formed in the upper face of the housing lower half 318. The blade 311 is accordingly deflected downwardly into the extended state as depicted in Figure 8. Figures 9 and 10 depict a fifth apparatus 410 for adjusting the trim of a marine hull 1. This apparatus 310 is similar to the apparatus 10 of Figures 2 and 3, except that the cavity 419 defined by the two halves 417, 418 of the housing 412 is tapered, being generally wedge shaped in cross section with a greater thickness toward the lower region of the main portion of the cavity 419. The tapered chamber, 419 reduces friction between the bladder 414 and the walls of the cavity 419 as the bladder chamber 413 expands during extension of the blade 411. Vertically extending and laterally spaced channels 426 are provided in the front and rear walls of the lower portion 419a of the housing cavity 419 so as to directly communicate the main portion of the housing cavity 419 with the water pressure acting beneath the hull 1. The external water pressure accordingly acts on the lower portion 414a of the bladder 414 to assist in retracting the blade 411 when the pressure supply 415 depressurises the chamber 413. A bulbous portion 420 is formed on top of the bladder 414 and is fixedly held in a corresponding recess 421 formed in the housing 412 so as to hold the upper portion of the bladder 414 in place in the same general manner as per the apparatus 10 of Figures 2 and 3. Figure 11 depicts a sixth apparatus 510 for adjusting the trim of a marine hull 1, with the blade 511 in the extended position. Similar to various of the earlier described apparatuses, the fifth apparatus 511 includes an integrally formed elastomeric blade 511 and bladder 514 housed within the cavity 519 of a two part housing 512 mounted on the hull transom 4. A bulbous portion 520 formed on the top of the bladder 514 holds the top of the bladder 514 in place within the housing 512. The bladder chamber 513 communicates with a pressure supply 515 via a conduit 524. In this apparatus, an elongate elastically deformable tie element 527 is formed within the bladder 514, extending between the upper edge 51 Ib of the blade and the upper wall 514b of the bladder 514. Here the tie element 527 is integrally formed with the blade 511 and bladder 514. The tie element 527 effectively acts as a return spring, assisting in retracting the blade 511 when the chamber 513 is depressurised. The tie element 527 is stretched beyond its natural state when the bladder 514 expands during extension of the blade 511, and the elasticity of the tie element 527 provides a restoring force serving to assist in retracting the blade 511 upon release of pressure within the expandable cavity 513. The tie element 527 may extend along the lateral length of the bladder 514, thereby dividing the chamber 513 into two sub-chambers communicated by way of an extension 524a of the conduit 524 extending through the thickness of the tie element 527. Alternatively, discrete tie elements 527 may be spaced along the lateral length of the bladder 514, with gaps therebetween. Vertically extending and laterally spaced channels 526 are provided in the front wall of the lower portion 519a of the housing cavity 519 to communicate water pressure with the front of the main portion of the housing cavity 519. Communication with the rear of the housing cavity 519 is achieved via laterally spaced apertures 528 extending from the channels 526 through the thickness of the blade 511. Figures 12 and 13 depict a seventh apparatus 610 for adjusting the trim of a marine hull 1, with the blade 611 in the retracted and extended positions respectively. The seventh apparatus 610 combines features of the fifth apparatus 510 of Figures 9 and 10 and the sixth apparatus 510 of Figure 11. The cavity 619 of the housing 612 is tapered as per that of the fifth apparatus 410 so as to reduce friction between the bladder 614 and the walls of the cavity 619 during expansion of the bladder chamber 613. An elongate elastically deformable tie element 627 extends between the blade upper edge 61 Ib and the bladder upper wall 614b as per the sixth apparatus 510 so as to assist in retraction of the blade 611 on de-pressurisation of the chamber 613. The bladder 614 is retained in position by way of a bulbous portion 620 at the top of the bladder 614 and the chamber 613 communicates with a pressure supply 615 by way of a conduit 624 as per the other described apparatuses. The blade 611 is provided with a stiffening element 629, formed of a more rigid elastomeric material, so as to assist in stabilising the blade 611 and ensuring that water pressure acting on the extended blade 611 does not excessively deform the blade 611, thereby reducing the effectiveness of the blade. Figure 14 depicts an eighth apparatus 710 for adjusting the trim of a marine hull 1, with the blade 711 in the retracted position. In this configuration, the bladder 714 is a more rigid structure compared to the collapsible bladders of the apparatuses discussed above. Particularly, the bladder lower wall 714a is sufficiently thick to be effectively rigid under operational pressures whilst the forward and rear walls 714c of the bladder act together with a central elongate elastically deformable tie element 727 to provide for expansion of the chamber 713 by stretching of the bladder forward and rear walls 714c and the tie element 727. On release of pressure from the pressure supply 715 in the chamber 713, the elasticity of the tie element 727 and bladder front and rear walls 714c act to draw the bladder lower wall 714a and blade 711 upwardly without collapsing of the bladder 714. The bladder lower wall 714a effectively reciprocates within the housing cavity 719 similar to a piston within a piston chamber. The relatively rigid thick bladder lower wall 714a provides additional stabilising support to the blade 711 beyond that provided by the guiding function of the cavity lower portion 719a. The top portion of the bladder 714 is retained in position by way of a bulbous portion 720 held within a recess 721 in the housing 712. This eighth apparatus 710 is a more heavy-duty unit suitable for larger vessels. Figure 15 depicts a ninth apparatus 810 for adjusting the trim of a marine hull 1, with the blade 811 in the retracted position. This apparatus 810 is designed to be mounted within a laterally extending recess 6 formed in the running surface 2 of the hull 1 forward of the transom 4. The recess 6 may be formed in the running surface 2 immediately forward of the transom 4, so as to have the same general effect as a transom mounted blade, or may be located further forward if it is designed to generate increased lift at any point further along the running surface 2. The ninth apparatus 810 is of a similar form to the fourth apparatus 310 of Figure 8, with an expandable chamber 813 being defined by an elastically flexible membrane 813 that is integrally formed with the blade 811 and the upper component 817 of a multi-component housing 812. The housing 812 is completed by two plastic wedge components 818. The wedge components 818 are secured to the housing upper component 817 by way of fasteners 825 with opposing free end portions of the membrane 814 fastened therebetween. A pressure supply 815 communicates with the chamber 813 via a conduit 824 and seacock 829 that passes through the hull structure. This seacock 829 forms the only possible point of water ingress into the hull 1, and can be closed so as to seal the hull 1 when not in use. Figure 16 depicts a tenth apparatus 910 for adjusting the trim of a marine hull 1, with the blade 911 in the extended position. This apparatus 910 is similar to the sixth apparatus 510 depicted in Figure 11 , except that the blade 911 is formed separately to the bladder 914 and secured thereto by way of a fastener 940. The separately formed blade 911 is formed of a rigid material, such as a metal or plastics material. Figures 17 and 18 schematically depict a pressure and control circuit for pressurising the bladder chambers of each of the above described apparatuses. A combined pump and pressure reservoir 15 is mounted in the hull 1. The reservoir is filled with an hydraulic fluid, with the pump component of the combined pump and reservoir typically being a DC electrical, low pressure, low volume pump. The reservoir/pump 15 , is wired to the vessel's electrical system by way of wiring 30. The reservoir/pump 15 is in fluid communication with the expandable chamber 13 of the bladder 14 by way of a conduit 24. An activation switch 31 is located on a control panel at the vessel helm. A pressure indicator gauge 32 communicates with the conduit 24 by way of a junction coupling 33 and a pressure conduit 34 to provide an indication of the system pressure. This system pressure serves as a guide to the degree of extension of the blade. Alternatively, a pressure transducer may be coupled to the conduit 24 and electrically coupled to the pressure indicator gauge 32. For vessels provided with two blade systems, two separate control circuits would typically be provided, as depicted in Figure 17. Alliteratively, for vessels with two or more blade systems on the same side of the transom, a seal control circuit may be interlinked between each of the blade systems on that side of the transom. Figure 19 schematically depicts an alternate pressure and control circuit which incorporates a manual back up system. A low pressure, low volume DC electrical pump 35 is located downstream of a fluid reservoir 15. The pump 35 is activated by an activation switch (not depicted), connected to the pump 35 by way of wiring 30. Wiring 30 connected to the activation switch also controls a DC electrical valve 36 located downstream of the pump 35. When the valve 36 is energised, when the pump 35 is also activated, the valve 36 closes an electric pump return loop 37 that bypasses the pump 35, enabling fluid to flow back into the reservoir 15 when the valve 36 is open upon deactivation of the pump 35. A back up hand pump 38 communicates with the reservoir 15, to enable pressurisation of the blade chamber 13 if there is a failure in the DC electric pump 35. A four way valve 39 is controllable to selectively isolate one or both of the electric pump 35 and the hand pump 38. A pressure gauge 32 communicates with the blade supply conduit 24 downstream of the four way valve 39 so as to provide an indication of the system pressure. The four way valve 39 has four positions as follows: all conduits 24, 41, 42, 43 closed, for maintaining pressure in the bladder chamber 13; blade supply conduit 24 open and DC electric pump conduit 41 open, for pressurising the bladder chamber 13 using the electric pump 35; blade conduit 24 open and return line conduit 42 open, for depressurising the bladder chamber 13; and blade conduit 24 open and hand pump conduit 43 open, for pressurising the bladder chamber 13 using the hand pump 38. Figure 20 depicts a further alternate pressure and control system that pressurises the chambers 13 of the bladder 14 of two blade systems by way of a manual hand pump 38 only. This circuit is provided with a fluid reservoir 15 and a five way valve 40. The five way valve 40 is controllable between the following five positions: all conduits 42, 43, 44, 45 closed for maintaining pressure in the bladder chambers 13; hand pump conduit 43 open and port blade conduit 44 open, for pressurising the port bladder chamb er 13 ; hand pump conduit 43 open and starboard blade conduit 45 open, for pressurising the starboard bladder chamber 13; return line conduit 42 open and port blade conduit 43 open, to depressuri.se the port bladder chamber 13; and return line conduit 42 open and starboard blade conduit 45 open, to depressurise the starboard bladder chamber 13. The person skilled in the art will appreciate that various features of any of the systems described may be utilised on other of the described systems. Various other configurations and modifications of the systems described will also be appreciated by the person skilled in the art.