final update...

Fitted the new part Sunday eve, also changed the green seal in the co2 cylinder and lubed with pellgunoil.

All working fine now.

I'd also suggest not filling the co2 cylinder to the full weight, as this puts more pressure on the threaded receiver, unlike a modern pcp if the thread does strip its not going to be easy to find a replacement, think I got very lucky with Protek having the part in stock. I'd also recommend not storing the pistol with a filled co2 cylinder screwed on.