Coventional boiler won't fire up - pilot ok.

the gasman is correct - changing the gas valve is indeed a job for a CORGI registered chap. The rule of thumb above is spot on - any work which involves breaking into any part of the gas line has to be carried out by a CORGI registered engineer.

However, it is often the solenoid coil which goes on these valves - if the solenoid on the valve is kaput, and can be changed without disturbing the gas connections, you can do this yourself.;)

As an electrician, I'm all for folk doing what they can to fix these problems themselves, within their capabilities. However the CORGI issue is slightly different - even if you are perfectly capable of doing the work, anyone doing so without CORGI registration should be aware that they are breaking the law.