Thanks everyone for all the really helpful advice. I phoned round a few places localish to me and the bloke at Portsmouth Gun Centre said bring it in and he’d have a crack at it. Incredibly when I got there fully expecting to have to drop it off, and then to go back a few days later to collect it, making it a 80 mile round trip to drop and collect, as he was quiet he said he’d have a go there and then. 20 minutes later, job done! A fully sealed S200 cylinder. A good chat about his previous experience as an air arms trained technician and a look at some of his wares on the shelves and he only charged me £20 which I thought was an amazing bargain….. I know where I will be taking my business in the future. He said they are a fairly difficult thing to do but they have a big old engineers vice with lead lined jaws to as not to mark the cylinder and the right tools so it makes it a bit easier.
This may help people who need a reasonable priced repair and don’t want to attempt it themselves.

Nemo