Thanks guys.
It is a 1989 RR 3.5EFI front axle so the shafts are 10/32.
I had a look at it again late last night after half a bottle of wine and think I have sussed it out.
I reckon the brass bush is held against the back of the swivel house by the lip on the old shaft, presumably this is to help reduce the lateral movement of the shaft. There is a circlip thing at the end of the replacement shaft which engages inside the CV itself (no circlip on the old shaft) to prevent lateral movement.
I am putting the old shaft down to the local garage so they can suss out a replacement and in the meantime I will stick the replacement shaft in to see what happens.
But as always it has started to snow up here in Aberdeen and the motor is in pieces, guaranteed to be gone by the time I get it fixed isn't it!
G.