Sigh. Told me what? You have *completely* missed the point. As I mentioned I'd already determined, with your help, that the problem was almost certainly the gas valve but given the intermittent nature of the fault the PCB wasn't out of the question. Hence, either way I was looking at a =A3100+ parts price. I didn't call in WB so much to find the fault, but to fix it (although there was still the possibility I'd got the diagnosis wrong).
The ethos being to DIY? In my case I needed a new gas valve fitting - safely and correctly adjusted. Not only do I lack the knowledge but also the equipment. Surely this is one such job that's acceptable to leave to someone else? If erring on the side of caution like this makes me a hairdresser then so be it.
Come on Geoff - your comments are not being very helpful, which is a shame given the enormous amount of help you gave me in the first place diagnosing exactly what was wrong.
Mathew