Do you mean the seal inside the female thread that screws onto the reg/rifle ??? If so its a 7x2 o'ring which is the same as Walther LG300 cylinders, any local O'ring supplier should be able to sell you a couple for next to nothing. Go for a nitrile in 90 shore but a 70 shore will do.
I have had nothing but excellent service from Harry Preston, but some people have given me the impression that he doesn't like to sell parts that he thinks need to be fitted by an expert!? I gather this seal is rather fiddly. I'll give him a ring and see what he says. Atb Alec.
Yet more first rate service from Harry Preston! After convincing him that I could remove the old seal and refit the new one he was happy to send me one straight away.
I stupidly forgot! and missed my chance to ask him about the distinct lack of sponsorship! Sod it, I knew I'd forgotten something!!