If everything else is working fine, then it sounds like a flow switch. If you can find your manual you will be able to find out where the flow switch is located. What it is is a magnet which is pushed one way when the water flows and the other when it isn't flowing; this operates a reed relay which turns on and off. If you can find where it is located, switch the mains off to your boiler, disconnect the wires from the flow switch to the main board and monitor the resistance with the water turned on and turned off. You should find it is open circuit with no water flowing and zero ohms with the water flowing. If it isn't doing this you need to turn off the water and see what's happening inside the pipe.
eduk, October 2013
