Not 100% sure, but think the lock barrel is held in by an expanding circular clip which fits around the rear of the barrel. It has to be compressed to pull the barrel out.
Your right Pete it is retained by a circular clip which slips into a "channel" on the barrel... the clip can be removed by prying it open as it than expands and can than be released from the barrel...
I can try 'n find mine and take a picture for you if you'd like some more info..
'68 Sunbeam Rapier fastback {B341006846 OD/LHO} '73 Sunbeam Rapier fastback {AAD 781863 } I like a car that looks normal but really knocks your socks off!!