Clutch out today.
Found the problem, the pressure plate that is supported by 3 straps has a strap location rivet failed and has allowed the plate to shift to one side, thus running eccentric to the clutch as a whole, no wonder the vibration was awful!
So, need a new heavy duty pressure/cover assembly, but I can only replace it with an identical design....
The damaged strap, the rivet on the right is cocked over, should be upright:
