Problem solved - took the easy way out.

Went to an auction today and picked up a sporterised LB No4 Mk1/3 in 7.62 which had the required swivel, + PH5C sights + An immaculate Sterling 7.62 magazine.

Stripped the rifle of all usable parts and told the auctioneer to keep the barreled action (shame - it was in very nice condition but I have no available slots on my FAC)
Stick the sights and magazine on ebay and the rifle is paid for and I get the swivel for 'free'.

Cunning plan ?