I have just had the same problem. Defect was circlip which holds microswitch block onto diverter valve diaphragm casing had corroded away thus the switch would only make intermittently. Managed a temporary fix using 22 swg stainless wire. I have previously replaced the diverter valve diaphragm (2 years ago) for same problem - the diaphragm rubber was perished. You can see the diverter valve arm operate the microswitch if you turn heating control to summer (off) and have someone run a hot tap. It's a small plunger with a black round plastic cap on it located in the middle of the black plastic block on top of the diverter valve and should rise to make the microswitch when you run a hot tap.this should fire the boiler when working correctly.The microswitch block will rotate freely if you can't see the diaphragm lever arm and you can hear the microswitch click when it makes. BE CAREFUL though, the microswitch is 240 volts so best isolate electricity before rotating the microswitch block!!! If the diaphragm plunger isn't moving, the diapragm itself is probably perished and can be replaced. The microswitch can be tested by manually operating it very carefully with an insulated small screwdriver or similar Good Luck.
Vince, February 2009
