FOOTWEAR This invention relates to footwear and in particular to footwear including a sole and a retention member to retain the sole on a user's foot. A common form of footwear comprises a sole including toe, instep and heel regions and a strap arrangement to retain the sole on a user's foot, which strap arrangement has a peg upstanding in the toe region so as to extend between the first (large) and second toes of a user, and a strap connected at least to one side of the instep region, but usually to both sides, and also being connected to the upper end of the peg. In use, the peg is located between the first and second toes and the strap extends over the upper part of the foot, so as to retain the footwear sole to the wearer's foot and to limit movement in the lateral sense of the sole relative to the wearer's foot. Such footwear is commonly referred to as a flip-flop and is very widely used, especially in warmer climes. Flip-flops tend to be relatively low cost footwear, possessing few if any aesthetically attractive design features. Further, many flip-flops have the most rudimentary soles, often comprising no more than a simple plate of a plastics or similar material of a uniform thickness and making no concessions to the contours and ergonomics of a user's foot. As such, flip-flops are generally regarded as low quality cheap footwear and are not recommended by podiatrists. This invention aims at addressing at least some of the shortcomings of known forms of flip-flop, to provide simple footwear which can be made to be highly effective, aesthetically pleasing and further may be beneficial for a wearer's foot. According to this invention, there is provided footwear comprising a sole including toe, instep and heel regions, and a retention member provided at the toe region which retention member upstands from the upper surface of the sole and is adapted in use for location between the toes of a wearer, the retention member having a significant lengthwise extent in the direction of the length of a wearer's toes and flaring outwardly at the upper end thereof so as at least partially to overlie those toes of a wearer between which the retention member is located, so that the footwear may be retained on the foot of a wearer by the interaction of the retention member and the wearer's toes. The footwear of this invention differs from conventional flip-flops in that no strap extends from the upper end of the retention member, over the upper surface of the foot and down to at least one side of the instep region of the sole. Rather, the retention member advantageously is shaped so as to fit closely to the toes between which that member is located, whereby the retention member utilises the natural shape of the foot in order to assist in levering the sole against the foot. Further, the retention member allows directional control of the sole, from the toes of the wearer. It is highly preferred that the retention member is moulded in one piece from a resiliently flexible plastics material or an elastomeric material, such as a foamed elastomer or a foamed natural or synthetic rubber material. The sole may comprise a moulding of a suitable grade of plastic or synthetic resin which also may be foamed for lightness, and the retention member may then be moulded integrally with the sole. Preferably however the retention member is formed integrally with a portion of the upper part of the sole and which defines the toe region thereof, the retention member blending smoothly into that portion. In this case, the remainder of the sole may be moulded from a different grade of plastics material from the retention member, whereby the remainder of the sole region may have characteristics better suited to ground contact and wear resistance while the retention member is softer and so kind to the toes and adjacent parts of the wearer's foot. Preferably, the retention member has two flanks disposed side-by-side with one flank blending into the toe portion to one side of the retention member and the other flank blending into the toe portion to the other side of the retention member. Each flank may have a rear edge facing the heel region of the sole, the rear edges of the two flanks being conjoined in order to give the retention member sufficient strength and shape stability. Preferably, the conjoined rear edges of the flanks flare rearwardly and in the upward direction, so that in use the upper ends of the flank rear edges partially overlie the joint region between the first and second toes of a wearer. Such a retention member construction facilitates gripping by the toes of a wearer, so assisting the wearer in maintaining contact between the sole and his foot and also giving directional stability of the sole with respect to the foot. Further to increase comfort in a case where the sole is moulded from a harder grade of a plastics material than the retention member, there may be provided an upwardly-facing insert in the heel region of the sole, which insert is moulded of a softer material and preferably of the same or a similar grade of material as the retention member. The sole may have a recess for accommodating such an insert, into which the insert is bonded. A wearer of footwear of this invention as described above may feel that the retention member insufficiently retains the sole in place against the wearer's foot. To address this, a strap may be provided for example to extend around the ankle region of the wearer, to enhance the attachment of the sole to the foot. Such a strap may be connected to both sides of the instep region of the sole and extend around the rear of the ankle. Preferably, the length of such a strap is adjustable in order to suit the wearer's foot. In addition, or alternatively, the strap may be resiliently extensible. Conveniently, such a strap may be connected to a moulded sole by providing a pair of spaced-apart slots through the sole in the instep region thereof, the strap passing through those slots and beneath the sole between the slots, thereby connecting the strap to the sole. By way of example only, one specific embodiment of footwear of this invention will now be described in detail, reference being made to the accompanying drawings in which:- Figure 1 is a diagrammatic general perspective view of the embodiment of the footwear; Figure 2 is a side view of the embodiment of Figure 1 ; Figure 3 is a longitudinal section through the embodiment, but with the strap omitted for clarity; Figure 4 is a plan view of the embodiment, again with the strap omitted for clarity; Figure 5 is an under plan view of the embodiment, including the strap; and Figure 6 shows the strap used in the embodiment. The embodiment of footwear shown in the drawings comprises a sole 10 having a toe region 11 , and instep region 12 and a heel region 13. The greater part of the sole 10 is moulded from a plastics material and typically from a foamed EVA. Such a material is relatively light and rigid, but also provides cushioning for a wearer's foot. The upper surface is profiled for comfort, and the underside of the sole is formed with grooves 14 to serve as a tread for the footwear, to assist grip between the sole and the ground when the footwear is in use. Moulded into the upper surface of the sole 10 are recesses 15,16, respectively in the toe and heel regions 11 ,13. Within the toe recess 15, the sole is slotted from the front, rearwardly towards the heel region 13. Fitted into recess 15 is a retention member 17 moulded from a foamed rubber material which is softer and less rigid than the foamed EVA of the sole. The retention member 17 is bonded in the recess by a suitable grade of adhesive. The edges of the retention member blend smoothly with the upper surface of the sole 10 around the edges of the recess, to ensure that there is no discomfort for the wearer. The retention member itself has a generally planar area 19 matching the upper surface of the sole 10 around the toe region recess 15. The retention member 17 defines a pair of upstanding flanks 20 within the slotted part of the sole, those flanks flaring apart in the upward direction away from the sole. The flanks are conjoined around their rearwardly directed edges, which edges are also curved back towards the heel region 13. Fitted into the recess 16 in the heel region 13 is an insert 23 moulded from a similar grade of foamed rubber to that of the retention member 17. As that material is softer than the foamed EVA of the sole, the heel insert 23 provides improved cushioning for the wearer's heel. To each side of the instep region 12 of the sole 10, there is provided a generally vertically extending slot 24, the underside of the sole instep region having a channel 25 (Figure 3) extending between the slots. A strap (Figure 5) is passed through the two slots 24 so as to extend along the channel 25 and thus be clear of the ground when the footwear is in use. The strap has a central portion 26 from one end of which extends a resilient first end portion 27 having a buckle loop 28 connected to the free end thereof. From the other end of the strap extends a resilient second end portion 29 provided with a tab 30 at its free end, that tab comprising one part of a hook and loop fastener, the other part 31 of that fastener being provided on the other side of the second end portion 29 or on the strap central portion 26. The first and second end portions 27 and 29 are formed from a foamed plastics material such as neoprene, for user comfort and to give some lengthwise elasticity to the strap so that it may stretch slightly in use. The central portion 26 of the strap is of a foamed polymer to give the strap sufficient rigidity for it to hold the shape shown in Figure 1 without the presence of a wearer's foot, while at the same time being sufficiently flexible for the strap to be comfortable, in use. As will be appreciated, the footwear is worn by a user inserting the retention member 17 between his first and second toes, so that the rearward edges 21 of the flanks 20 are more or less in contact with the web between the toes. As the retention member is resiliently deformable, it will deform as required closely to fit against the contacting toe regions. Because the flanks flare outwardly in the upward direction, those flanks also contact the upper portions of the toes. Further, in view of the length of the flanks in the lengthwise direction of the sole, the flanks will serve to resist lateral movement of the sole with respect to the wearer's foot. The strap extends around the rear of the wearer's ankle and is adjusted to suit the wearer's foot. To achieve this, the second end portion 29 is passed through the buckle loop 28 and tab 30 is then connected back to the other part 31 of the hook and loop fastener, with a light tension in the strap. Once adjusted, the footwear may be removed from a wearer's foot or replaced thereon without the need to release or effect further adjustment of the strap, having regard to the resilience imparted by the first and second end portions 27 and 29. The footwear as described has no strap extending from the upper region of the part which is located between the first and second toes of the wearer. As such, the shoe is very comfortable for the wearer, who may lever the sole against the underside of the foot by suitable action of the toes. However, in view of the presence of the ankle strap, the wearer equally may simply relax his toes and rely on the strap urging the sole rearwardly to maintain good contact between the retention member 17 and the first and second toes.