Only just seen your replies John - many thanks.
I took the pistol apart tonight and the piston head o-ring is completely trashed (comforting that it was so obvious!) - more on this post:
http://www.airgunbbs.com/forums/show...7&postcount=10
I'd already ordered a seal kit when I saw your post (had a feeling that the o-rings would be standard parts, but the kit was only £16 from BAR, so not too bad). As you say, I'll sort out some spares from an engineering supplier to keep for the future!
thanks again
Matt