Thanks Martin,
Have already tried IPA, but the rubber is extremely hard & cracked. It will work for 10 minutes or so after cleaning, but then slips badly. Have also previously adjusted the clutch boss, but has to be set too tight for the clutch to have any grip at all.
I had a sudden idea, that the hose union washers , (as used on appliance taps) might fit. However their O.D. is 3/4" & too small .
Have just measured the O.D. of the originals, at 30mm.
It would be good to be able to finish off the job properly, I have already sorted out all the various electronic faults & re-polished the cabinet, so just this bit to go!
If you can lay your hands on a good used clutch, or just the rubber facings, that would really be appreciated.
Thanks for your time, 
David.