If Abyss-UK are still on the go then they used to sell Thermo valves. I have a couple of US Luxfers and they both have Thermo valves in them. I am in the same situation, not BS or EN stamped so they can't be stamped after a test. I get my local shop to test them and just give me a verbal OK on the understanding that a. they don't get a stamp or cert and b. they never turn up in the queue for filling. I have to admit, I do sneakily break rule B
Anyhow, stop being such a tight git and buy an adaptor, they are only a few quid. Bodging does the job but I regularly shite myself in my garage using my Acme Decanting Kit, fettled from the guts of a MDE manifold with a couple of assorted adaptors on each end. No bleed screw, just undo the A-clamp slowly until the pressure releases -- either in the o-ring or your sphincter!!!
