Any plumbers or boiler experts out there?

My Baxi combi boiler is not working - a warning light comes on and looking at the manual and online it said it was either fan, flue or pcb problem. A plumber has now been out and replaced the pump, which did not fix the problem. He has ordered another part - no idea what. He is coming tomorrow to put this part in but cannot guarantee it will work - he says he might have to replace the pcb. The question is, can he just keep charging. I can get British Gas to repair the boiler for a guaranteed �168 but this plumber says he can't guarantee his charges as the part tomorrow might not be right. Can he charge when he has not fixed the problem or am I within my rights to get someone else? Boiler was fitted this plumber roughly 4 years ago.