This invention relates to a process for making toothbrushes, in particular an injection moulding process. Toothbrushes are well-known articles and normally comprise a head and a grip handle, generally with a narrowed neck region in between, all arranged along a longitudinal head-handle direction. Normally toothbrushes are made primarily of a rigid plastic material, e.g. a polypropylene or styrol acryl nitrol ("SAN"). Many are so called two-component toothbrushes comprising a part made of plastic, e.g. the head and the main structure or "skeleton" of the handle, and a part made of a softer resilient elastomer, such as a thermoplastic elastomer e.g. Santoprene™ e.g. comprising part of the grip handle, to enhance grip, aesthetic appearance or to influence flexibility. Such toothbrushes are disclosed for example in US-A-5,054,154 and EP-A-O 336 641 among others. Toothbrush heads generally incorporate oral hygiene parts such as bristles, which project from the head in a bristle direction. Toothbrushes are normally made by a process in which the plastic material part of their structure is first made, generally by injection moulding. Then this plastic part is enclosed in a mould cavity which defines the shape of the elastomer material part, and the elastomer material of the second component in a fluid state is injected into this mould cavity to form the elastomer material part. Normally the plastic material is formed with one or more cavity to receive this second material. US-A- 3,633,237 discloses such a toothbrush head having cavities in the plastic material part with which the elastomer part can engage. Normally such injection moulds are mould assemblies comprising one or more, generally two, mould blocks each with a part mould cavity therein, which when brought together in the assembled mould mate so that their part mould cavities combine to form a complete mould cavity between them. WO- A-02/13656 for example describes such a process and suitable moulding machinery for its performance being a so called stack mould. US-A-6,353,958 discloses a toothbrush incorporating a plastic spring part made of a higher elastic modulus than that of the toothbrush handle, and discloses that such a toothbrush may be made by inserting the spring part in the mould and then injecting the plastic material of the handle into the mould. US-A-2004/0113312 discloses an injection moulding process in which a first component is made by injection moulding, and is then enclosed in a second injection mould for injection of further plastic material. It is an object of this invention to provide an improved process for making a toothbrush comprising at least two rigid plastics materials and optionally an elastomeric material. Other objects and advantages will be apparent from the following description. Accordingly this invention provides a process for making a toothbrush, wherein; in a first stage a toothbrush part is made of a first plastic material, having a conduit therein which is open at one or more opening on the outer surface of the part, in a second stage a second plastic material in a fluid state is injected into the conduit via an opening, and this second plastic material is allowed to harden. The toothbrush part may be a toothbrush handle, and the first and second plastic materials may be any of the plastic materials known for toothbrush handle manufacture. Preferably the first plastic material is a transparent, optionally coloured, material so that the second plastic material is visible through the first plastic material, providing an aesthetic function, hi such a case preferably the second plastic material may be an opaque, coloured or translucent material. The first and second plastic materials may be the same plastic material but of different colours, or one transparent and the other coloured or filled with an opaque filler. Preferably the process is an injection moulding process wherein: in the first stage a first mould is provided having a first mould cavity, bounded by a cavity wall, which defines at least some of the shape of the toothbrush part, the mould incorporating a core which can be moved between extended and withdrawn positions in which respectively the core extends a further and a lesser distance into the mould cavity and which in the extended position defines at least part of the shape of the conduit, providing said core in the extended position, injecting a first plastic material into the mould cavity in a fluid condition and allowing the first plastic material to harden, moving the core into the withdrawn position, then in the second stage injecting the second plastic material in a fluid state into the conduit and this second plastic material is allowed to harden In this moulding process, preferably there are plural cores which in their extended position define the shape of the conduit. Preferably in their extended position plural cores meet to define a continuous path of the conduit. Such a core or plural cores may be provided by known technology in the injection moulding field injection moulding of providing parts which can be slideably moved into extended and withdrawn positions, with a suitable fluid plastic material- tight seal between the core(s) and the mould, e.g. a sliding seal. Preferably for the second stage the toothbrush part formed in the first stage is enclosed in a second mould having a second mould cavity, bounded by a cavity wall, which defines at least some of the shape of the toothbrush part. The second plastic material may be injected into such a second mould via an injection port which is in communication with the conduit when the toothbrush part is enclosed therein. However a first mould may be constructed in which it is feasible to inject the second plastic material via an injection port which is in communication with the conduit when the toothbrush part is enclosed in the first mould. The second mould cavity may have one or more recess in its cavity wall, which when the toothbrush part is enclosed in this second mould is/are in communication with the opening(s) at the outer surface of the toothbrush part. Flow of the second plastic material from the conduit through such openings into such a recess can result in formation of a convex bulge of the hardened second plastic material beyond the outer surface of the toothbrush part. Such a convex bulge may have a technical function e.g. in enhancing a user's grip of the toothbrush handle, or may have an aesthetic function. Preferably the first and second moulds each comprise at least two part-moulds which can be separated across a split line to enable removal of the formed toothbrush part therefrom. The conduit is a void in the hard plastic material, suitably in the handle and/or in the neck defining a path along which fluid material may flow. Preferably the conduit is continuous. The conduit maybe elongate and undulating in the longitudinal direction of the toothbrush. Preferably the conduit is open at plural openings on the outer surface of the part of the toothbrush. Preferably when there are plural openings, such plural openings are disposed on opposite sides of the part. Preferably such opposite sides are distanced in the bristle direction. In one form of this process the toothbrush part may incorporate a cavity, and in a third stage an elastomeric material in a fluid state may be injected into the cavity and allowed to harden. Preferably for such a third stage the toothbrush part formed in the first or second stage is enclosed in a third mould having a third mould cavity, bounded by a cavity wall, which defines at least some of the shape of the toothbrush part. The elastomeric material may be injected into such a third mould via an injection port. The third mould cavity may have one or more recess in its cavity wall, which when the toothbrush part is enclosed in this third mould is/are in communication with the injection port. Flow of the elastomeric material into such a recess can result in formation of convex bulges of the hardened second plastic material beyond the outer surface of the toothbrush part. The elastomeric material may have a technical function e.g. in enhancing a user's grip of the toothbrush handle or its flexibility, or may have an aesthetic function. Conventional plastic and elastomeric materials as used in the manufacture of toothbrushes may be used in the process of this invention. The invention also provides a first mould for a process as described herein, having a first mould cavity, bounded by a cavity wall, which defines at least some of the shape of a toothbrush part to be formed therein, the mould incorporating a core which can be moved between extended and withdrawn positions in which respectively the core extends a further and a lesser distance into the mould cavity and which in the extended position defines at least part of the shape of a conduit to be formed in the toothbrush part, providing said core in the extended position, the mould having an injection port for injection of a first plastic material into the mould cavity in a fluid condition. Preferred features of such a mould are as indicated above in relation to the process of the invention. The invention also provides a toothbrush when made by process of this invention. The invention will now be described by way of example only with reference to the accompanying figures which show. Fig. 1 a section through a first mould with cores extended. Fig. 2 a section through the first mould with cores extended, and with first plastic material in the mould cavity. Fig. 3 a section through the first mould with cores withdrawn, and with a formed toothbrush part in the mould cavity. Fig. 4 a section through the toothbrush part as formed. Fig. 5 a section through a second mould with the formed toothbrush part in the mould cavity. Fig. 6 a section through the second mould with first plastic material injected into the conduit. Fig. 7 a section through the toothbrush part as formed. Fig. 8 a section through a third mould with the formed toothbrush part in the mould cavity. Fig. 9 a section through the third mould after injection of elastomer material. Fig. 10 a section through the toothbrush part as formed. Fig. 11 a section through an alternative construction of second mould. Parts shown in Figs. 1-9 are listed below: 10 first inj ection mould 1OA, 1OB part moulds 11 split line 11 12 first mould cavity 13 cavity wall 141,142,143,144 plural moveable cores 151, 152, 153 154 guides 16 first plastic material 17 injection port 18 conduit 19 gaps 20 toothbrush part 21 head 22 grip handle 23 neck region 24 bristle holes 30 second mould 30A,30B part moulds 31 split line 32 second mould cavity 33 cavity wall 40 toothbrush part 34 second plastic material 35 injection port 36 plural recesses 110 openings 41 convex bulges 42 cavity 50 third mould 50A,50B part moulds 51 split line 52 third mould cavity 53 cavity wall 60 toothbrush part 54 elastomeric material 55 injection port 56 recess 70 alternative second mould Referring to Figs. 1, 2, 3 and 4, an injection mould 10 is shown, comprising two part moulds 1OA, 1OB which separate at split line 11. The mould 10 is made of conventional steel. The mould 10 defines a first mould cavity 12, each part mould 1OA, 1OB defining part of the cavity 12, the cavity 12 being bounded by a cavity wall 13. The cavity 12 defines the shape of a toothbrush part 20 to be formed therein. The mould 10 incorporates plural moveable cores 141, 142, 143 144. As shown in Fig. 1 these are in an extended position in which the cores 141, 142, 143 144 extend a further distance into the mould cavity 12. The cores 141, 142, 143 144 are slideably mounted in guides 151, 152, 153, 154 passing through the part moulds 1OA, 1OB, with a fluid-tight seal between the respective cores 141, 142, 143 144 and the guides 151, 152, 153, 154. In this position the cores 141, 142, 143, 144 contact each other or the cavity wall 13 opposite their respective guide 151, 152, 153, 154. As seen in Fig 2, with the cores 141, 142, 143 144 in this extended position a first plastic material 16 is injected via injection port 17 into the mould cavity 12 in a fluid condition and is allowed to harden. As seen in Fig. 3, the cores 141, 142, 143 144 are moved by sliding in their respective guides 151-154 into the withdrawn position in which the cores 141-144 extend a lesser distance into the mould cavity 12. It is seen that as the cores 141-144 are withdrawn they leave a conduit 18 in the hardened plastic material 16. Fig 3 A is a cross sectional view through line A-A of Fig. 3 showing how the core 141 does not extend across the entire cross section of the mould cavity 12, but leaves gaps 19 around each side of the core through which the plastics material 16 may flow. The mould 10 may then be opened at the split line 11 and the formed part 20 removed in a conventional manner. It will be appreciated that the cores 141-144 may have other orientations, for example they need not contact each other, but may contact only the cavity wall 13 opposite their respective guide 151, 152, 153, 154, for example the cores 141-144 may extend parallel to each other across the mould cavity 12. The product 20 of this first stage of the process is shown in a longitudinal section in Fig. 4. This product 20 is a toothbrush part comprising a head 21 and a grip handle 22 with a narrowed neck region 23 in between, all arranged along a longitudinal head-handle direction L-L (which may be a straight line, curved or undulating). The head 21 has small conventional holes 24 formed in it for subsequent insertion of bristles (not shown), which will extend in bristle direction B. The conduit 18 is shown, following a continuous elongate path, undulating in the longitudinal direction A-A of the toothbrush. The conduit 18 is open at plural openings 110 on the outer surface of the grip handle 22 of the toothbrush part. These plural openings 110 of the conduit 18 are disposed on opposite sides of the handle 22 distanced in the bristle direction B. Referring to Figs. 5, 6 and 7 in a second stage of the process the toothbrush part 20 formed in the first stage as above is enclosed in a second mould 30, comprising two part moulds 30A, 3OB which separate at split line 31. The mould 30 is made of conventional steel. The mould 30 defines a second mould cavity 32, each part mould 30A, 3OB defining part of the cavity 32, the cavity 32 being bounded by a cavity wall 33. The cavity 32 defines at least some of the shape of a toothbrush part 40 to be formed therein. As seen in Fig.6 a second plastic material 34 is injected into the second mould 30 via an injection port 35 which is in communication with the conduit 18 when the toothbrush part 20 is enclosed therein and is allowed to harden. The second mould cavity 32 has plural recesses in its cavity wall 33, which when the toothbrush part 20 is enclosed in this second mould 30 are in communication with the openings 110 of the conduit 18. Flow of the second plastic material 34 from the conduit 18 through such openings 110 into these recesses 36 result in formation of convex bulges 41 of the hardened second plastic material 34 beyond the outer surface of the toothbrush part 40. The toothbrush part product 40 of this second stage is shown in Fig. 7. The first plastic material 16 is transparent, and the second plastic material 34 is opaque, so in the formed part 40 the undulating shape of the second plastic material occupying the conduit 18 can be seen, presenting an aesthetic feature. Referring to Figs. 8, 9 and 10, the toothbrush part 40 from the above second stage incorporates a cavity 42. hi a third stage the toothbrush part 40 formed in the second stage is enclosed in a third mould 50, comprising two part moulds 50A, 50B which separate at split line 51. The mould 50 is made of conventional steel. The mould 50 defines a third mould cavity 52, each part mould 50A, 5OB defining part of the cavity 52, the cavity 52 being bounded by a cavity wall 53. The cavity 52 defines at least some of the shape of a toothbrush part 60 to be formed therein. As seen in Fig. 9 an elastomeric material 54 is injected into the third mould via an injection port 55. The third mould cavity 52 defines a recess 56 between its cavity wall 53 and toothbrush part 40 enclosed in this third mould 50 which is in communication with the injection port 55 such that flow of the elastomeric material into this recess 56 results. The elastomeric material 54 has a technical function in enhancing a user's grip of the toothbrush handle 22. Thereafter the mould 50 may be opened at split line 51 and the toothbrush part 60 may be removed therefrom in a conventional manner. Fig. 10 shows a longitudinal section through the toothbrush part product 60, in which the continuous path of the conduit 18 is occupied by hardened second plastic material 34, with convex bulges 41 of the second plastic material 34 adjacent to the openings 110 and disposed on opposite sides of the toothbrush handle 22, distanced in the bristle direction B. It is seen also that the conduit 18 extends longitudinally along the toothbrush handle 22 so that exposed parts 43 of the second plastic material are present. This product 60 may be conventionally further processed, e.g. bristles may be inserted into holes 24. Referring to Fig. 11 an alternative construction of second mould 70 is shown in which the product 20 may be enclosed, and provided with injection ports 35, 55 for respective inj ection of second plastic material into conduit 18 and of elastomeric material into recess 56.