Thanks for the suggestions.
I have noticed that there is a bit of play in the freewheel - it's firmly screwed
on to the hub, but there seems to be play between the freewheel body and the
sprocket assembly. It's not a lot, but it's noticeable. Could that be the problem?
The cable is a bit loose in the highest gear, but not very.
It's an old campag SR mech, but it was in good condition when I bought it,
not worn out, and the tension spring seems ok. It's not possible to do the daft thing of threading the chain round the outside of the chain guide (Which I have done with modern shimano derailleurs
So is the play in the freewheel likely to be the problem? And if so, is there anything I can do short of replacing it altogether?