Whats the probem with my Stelrad Group - Ideal Sprint F Combi?

The problem is with the hot tap water side of the boiler, (the central heating works fine all the time). When a hot water tap is turned on the boiler fires up, (or increases the gas if the central heating is already on), then runs for 2 or 3 seconds and then turns the gas, (or extra gas), off. It then cycles through this by starting up, (or increasing the gas), and stopping, (or reducing to just the central heating amount of gas).

So in essence once the water is turned on and the boiler fires up there is something monitoring the system and shutting down the tap water heating part only. Any suggestions would be greatly appreciated.