I'd always assumed that the problem was our mid-position valve getting stuck, however, the problem has been getting so frequent that I've stayed at the boiler to monitor the behaviour and I am hearing relays triggering but the boiler does not always attempt its ignition sequence and may give up at any stage - even immeidately after successful ignition. Thus I'm convinced it's the PCB and rather than spend a huge amount on a replacement, I'm spending £14.52 inc VAT P&P to get 3 replacement relays first. Since the boiler is 12 years old, the relays could easily have worn out contacts by now! They are OMRON G6RN 12VDC - available at suppliers including RS amd Rapid Electronics.
Roderick
May 2009