The problem may not necessarily be associated with the gas valve. You will need a corgi registered engineer to repair this fault.It may be the boiler overheat thermostat that could indicate a flue problem (if you have a conventional flue then it is possible that products of combustion may be escaping and therefore extremely dangerous).
My advice is to call a corgi registered engineer urgently and not to use the boiler until the problem has been resolved.
Good luck
Steve
May 2007