I have come accross this too myself.....
1st thing to do is change the clutch cable as the adjuster seem's to seize and eventually stick solid. what i would also do while the old cable if off is the ratchet which sit's on the clutch arm as this wears and crack's. this is under the cover on the side of the box (green on mine) and cost me about a tenner last time. This is a common fault so I was informed and is more easy to replace while the cable if off as it takes 2 people to reclip the cable when you wan't to remove again.