I had a similar problem some years ago. the boiler would always start in the morning but then tripped later in the morning. the solution, given to me as a possibility by a service engineer:and it worked: was to change the pump start capacitor that was being partially shorted by the higher temps. after the boiler warmed up but was ok when cold. they are cheap, if you can find one so worth trying. it is a cylinder screwed to the fan casing (this is one connection) with a single wire connected elsewhere: I cannot remember where: this being the other connection. Good luck
fellow sufferer, January 2009