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1. A prefilled syringe comprising a phial (11) containing a measured quantity of an injectable product, a fixed wall (12) at one end of the phial to receive a needle, a displaceable wall (14) at the opposite end of the phial and displaceable into the phial by means of a plunger (5) to expel the product out of the phial through the needle, and a two-part housing (1,2) for the phial and plunger in which the two parts are connected by interengaging screw threads (3,4) and having a stop surface on one housing part co-operable with a corresponding surface (7) on the plunger to limit displacement of the plunger (5) into the phial (11), wherein the position relative to the phial of the stop surface can be varied by rotating the two housing parts (1,2) relative to one another, thus controlling the distance by which the plunger (5) can be displaced into the phial (11), characterised in that the two housing parts (1,2) are provided with a click-stop mechanism (18) that permits them to be rotated relative to one another in only one direction and in such a way that free rotation is possible over a certain angle whereupon further rotation can only be achieved by passing a detent, and further characterised by a graduated dosing ring (19) mounted coaxial with, adjacent to, and rotatable relative to both housing parts (1,2), the dosing also being provided with means to co-operate with said click-stop mechanism to permit click-stop rotation of the dosing ring.
  2. A syringe according to Claim 1 wherein the first housing part (1) holds the phial and the second housing part (2) incorporates the stop surface (8), so that relative rotation of the two housing parts (1,2) enables the position of the stop surface to be varied relative to the first housing part.
  3. A syringe according to Claim 1, wherein the means permitting relative click-stop rotation of the two housing parts in only one direction comprises pawl means (18) on one of the housing parts.
  4. A syringe according to Claim 3, wherein a plurality of spaced detents (17) is provided around the periphery of the other of the housing parts to co-operate with the pawl means (18).
  5. A syringe according to Claim 4, wherein ten spaced detents (17) are provided.
  6. A syringe according to Claim 3 in which the pawl means (18) is on the first housing part (1), which is the outer housing part, and within which the second housing part fits, and around which is the dosing ring (19), wherein the second housing part (2) has a series of spaced longitudinal ribs or grooves (17) around its outer periphery, and wherein the dosing ring (19) has a corresponding series of spaced longitudinal grooves or ribs around its inner periphery, and wherein the grooves or ribs on both the second housing part (2) and the dosing ring (19) co-operate with the pawl means (18) to permit independent click-stop rotation of both dosing ring (19) and second housing part (2) relative to the first housing part (1), the pawl means (18) being shaped to permit the second housing part (2) to turn only in the direction which causes it to be screwed into the first housing part (1).
  7. A syringe according to Claim 6, in which the dosing ring (19) may turn in either direction relative to the first housing member (1).
  8. A syringe according to any preceding claim, in which the injectable product contains a pharmaceutically-acceptable preservative.
  9. A syringe according to any preceding claim, in which the phial (11) is transparent and an opening or a transparent window is located in the wall of the housing (1) close to the needle to permit the user visually to check the number of doses remaining in the phial (11).