I've got a problem with my Baxi WM51/3RS central heating boiler.
Everything seems to work fine but after around 5-10 minutes the boiler
stops burning and the whole system goes cold. Turning the controls
off and on after a couple of hours will cause it to burn again for
another 5 minutes, but it then shuts down again - otherwise the boiler
won't burn again. Pilot light stays lit.

From what I can tell, the pump, valve, themostat and controls all seem
to be working normally. When the boiler is actually firing, the
radiators will start to get hot and the pipe into the hot water tank
also gets hot, which suggests everything is ok.

My first guess was the boiler thermostat but replacing this hasn't
made any difference.

Second guess was a blockage or air lock causing the heated water not
to be leaving the boiler (thus triggering the stat), so I drained the
whole system (boiler included) and refilled it. Same problem.

Not sure how old the boiler is but I'm fairly sure its over 10 years
(as Baxi couldn't find it on their list and hence refused to send an
engineer out!). I'm beginning to accept I may just need to buy a new
boiler but don't want to do this and then discover the problem was
elsewhere!

System is a straight forward setup, conventional boiler, single
motorised valve, digital room thermostat and digital controls. Pump,
valve, thermostat and controls are all less than 2 years old and
everything was working fine up until this week.

Going to try bypass the room thermostat tonight and see if that makes
any difference (but I'm no electrician so may not have much success).

I'd be grateful for any ideas.

Cheers,
Steve