I joined up with Advanced craft City and Guilds Plumbing and was told I would have to redo my trade, I joined the Army for a bit of someting different so I changed my trade and went Chippy, not saying that your skills learnt in civvy street wont be used, mine were on many tasks and I was still a B3 knocker with no A trade so to speak,But then that was a while ago ,hope my ramblings help
It depends on your Quals. Gas is a class 1 task among other things, you still need to be a shoite mechanic. If you hold the necessary quals, you should get an assesment period at Chatham followed by an abridged course to fill in the gaps. However you need to see the careers and get them to flag it up.