In the preferred embodiment the high pressure chamber communicates with the injection orifice through a delivery chamber, the high pressure fluid from the high pressure chamber being supplied to the delivery chamber through a non-return delivery valve, the non-return delivery valve being operable to close the delivery chamber and maintain in the delivery chamber a charge of fluid stored under pressure. Preferably the non-return delivery valve has a valve member having a first stage of movement in which it moves to stop communication from the pressure chamber to the delivery chamber and a second stage of movement in which the valve member after having completed its first stage of movement allows limited pressure relief in the delivery chamber so as to thereby reduce the fluid pressure upstream of the injection valve.

The piston means is preferably movable under the action of the externally applied fluid pressure against the action of a main spring, the force applied by the main spring at least in part determining the externally applied fluid pressure necessary to initiate movement of the piston means, the injecting apparatus further including a delivery spring against the action of which the injection valve moves to allow fluid injection through the orifice, the strength of the delivery spring determining at least in part the pressure of fluid in the high pressure chamber necessary to open the injection valve to allow fluid injection through injection orifice.