water heating?

I have a myson medway super, multipoint water heater.when the hot taps are tuned on the boiler normally fires and the water flow reduces to get hotter. currently,when a tap is turned on the boiler fires but pressure of water flow stays high and not hot. i remember replacing a part some years ago, i believe that it may have been a diaphraghm when this problem occured previously. can anyone tell me what part i need to order. thanks in anticipation.