Jon,
Before doing anything else, try washing the clutch plate in petrol. To do this, block the drain hole in the bottom of the bellhousing, then pour about a pint of petrol in through the clutch inspection hole. Then lift the clutch and operate the kickstart about 20 times - this will spin the drive plate and wash away any oil that might have contaminated the plate. With this done, remove the plug and drain off the petrol. Finally, make sure you re-grease the thrust bearing - about 3 shots should suffice. This dodge has worked successfully for a lot of owners.
If you have to resort to replacing the clutch plate, the only source of supply is the Club spares department, but they probably only stock the plate as a kit of component parts to be assembled by the buyer!
Regards,
Eddie.