Hi Handy The gas vale (inc seleniod) is the large metal box the gas pipe leads into with the gas tap to turn the gas off. I have the sam boiler and the same thing happend to mine. Turned out to be the selenoid. easy to fit. it screws onto the front of the valve. normaly retail about £80 I was told....
I believe from a double glazing sales person that you can contact your local council building services, and they will come out to inspect and issue a certificate.
This service is not free, and I do not know what each council would charge.
