Hi Shaun,
I've a similar problem today. It turns out to be the OMRON power relay (miniture switches) located in the pcb. You'll get the spare from CPC. If you're not familar with the soldering iron, get a tv or video engineer to do it. It's much cheaper than your gasman. He probably ask you to get a new boiler. simongoh15 h/mailcom
simongee, November 2009