Found this page searching to an answer to the exact same problem that I have! I hope Bri has fixed his by now. In my case I've isolated the fault to the diverter valve behind the hot water cylinder. Seems its sticky and a bit of pressure on the actuating arm completes the elec circuit for the boiler to start up.
Can I just replace the diverter valve with a new one? Is that a diy job? Would I need to drain the system?
Richard M
September 2007