I found a long term fix:
Had the same problem with our 147 JTDm 3 months a go.
Stiff clutch that came and went, notchy pedal feed back, until it became almost impossible to depress the clutch; went on for about 2 weeks before I got around to looking at it.
After reading most of the opinions on the forum I came to the conclusion nobody really knew the cause or fix, so I went about finding the cause by systematically replacing components.
Started with the slave cylinder...clutch went light for a day or two, then back to stiff. Not the slave.
Next…I replaced the clutch master cylinder, the notchy pedal feel and squeak went away...clutch was a bit less stiff...but after a day or two it went back to being really stiff.
Next…I removed the clutch-actuating arm from the top of the gear box, pulled the top bearing out, cleaned and greased it with Finish Line PTFE mountain bike grease, miles better but not perfect.
Next…I removed the clutch-actuating arm again, pulled the top bearing a ran some extreme wet weather mountain bike chain oil down over the top of clutch release shaft to reach the bottom bearing, popped the clutch release top bearing and actuating arm back together, re-fitted the slave cylinder.
Fixed…Now its lighter than my BMW not quite as light as the GTA and has stayed like that for more than 3 months.
I think that every time the clutch actuating-arm was disturbed, it freed the mechanism off for a short while. I think the same thing happens when people get the clutch changed, the release mechanism gets disturbed and/or lubricated as well!!!
Try it… Battery box out…clutch actuating arm off, top bearing off, bit of oil, bit of grease, back together in a 1-2 hours
Hope this helps