How do you replace a Gas Valve on a 240 Worcester combi?

It may sound like a dumb question, but this is gas!

Please reply with a suitable URL with a set of rules and to do and not to do, in order to replace it properly and do a good job or is there a way of refurbishing it? I think a Gas Valve should be refurbishable as I bet it only needs opening and cleaning!

I can hear the electrical relays clicking fine.

Thanks