Thanks for the advice. After trying several options I found a solution. The generic BSA worked reasonably well but had a tendency to occasionally pull out when opening the breech. I also tried a Weihrauch seal and with a bit of sanding on the rear face that too seemed to work but still wasn't quite there.
Then I noticed TR Robb listed the 335 breech seal - ordered one up and problem is solved. Now I know it works I'll order up a couple of extras to put away.