Well I called Protek, not really expecting them to have this piece, got a call back and was amazed that they’d found a brand new one. Collected today at Kempton along with some green co2 cylinder seals.

So should be up and running later tonight, after top gear!

FYI I found a caution message on a German airgun site to ensure that the seal and threads are kept lubricated to prevent the brass threads stripping

Big thank you to Protek and the advice offered here.