Bob
I used a 14 mm across flats bolt in the valve, with nuts double locked on the thread, and a big socket wrench, they did turn, and it would have been better with the nuts welded.
My other problem was the hex on the end of the cylinder had been butchered so I had to clamp that in a vice.
But I did get it out with a touch of warmth.
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?