When I turn the tap on the boiler fires up, correct lights appear and I get hot water. then the Lights go out in sequence (1) Fan, (2) Ignition and (3) Flame. and the water goes cold - very annoying when in the shower. Then the lights come back on again in the same sequence and hot water reappears. This continually occurs. I have witnessed the microswitch working on the diverterer valve when hot water is turned on so that seems ok. ANY IDEAS?