Paul,
The clutch mechanism has three rollers that run up ramps. They sometimes get stuck so that they don't easily run back. Try removing the inspection cover on the nearside of the bellhousing - inside you will see one of the rollers - try applying a little oil to the roller and to the top of the cam (this will eventually drain around the cam and lubricate the other 2 rollers). Also check that the clutch cable is fitted with a return spring - this is just a light compression spring fitted over the cable between the cable adjuster and the operating lever to assist it's return. Even without the lever self-returning, you should be able to put the bike into gear, lift the clutch and then rock the bike to and fro to make sure the clutch actually frees - the plate could just be stuck due to lack of use. The gears are one up, three down.
If the clutch still doesn't free, it may be fitted with early generation asbestos-free linings which have been known to give off dust that tended to jam the 6 sliding pins through the flywheel. In this case, dismantling and a good clean is the only option.
Regards,
Eddie.
