Thanks for your response ChrisR.
We have had problems with one of the Vaillant boilers for some time now. Basically, the flame reset switch on the front of the boiler would continually pop out (varying from every few hours to every few days) causing the boiler to shut down. As both of the boilers are in the attic, sometimes we didn't realise it wasn't working. Having already spent £1,000 trying to get the boiler repaired with one CORGI plumber, he couldn't fix it (even though he wanted to replace part after part) and the boiler wouldn't even fire-up after he had finished. I recently took out a maintenance contract on the boiler to reduce some of the costs. The company replaced the flame supervision circuit board, and the boiler fired-up and appeared to work without fault. A few days later, the reset switch started to pop out again. The maintenance company came back and diagnosed it as a air-pressure switch fault. The part was ordered and they came yesterday to fit it yesterday. Once fitted, they turned on the boiler and it blew the flame supervision circuit board, again! They tell me the fuses are ok, there is 240v coming to the boiler, but there is no output to the ignition device. Vaillant won't come out because of inadequate access/no hand-rail. If this boiler can't be repaired, I guess the only way forward is to replace it and as it would be a condensing boiler, it would make sense to do both boilers.
Any ideas?
Thanks