Aye Dave,
I would take all the plates and clean them in a bath of brake cleaner (5 litre tin from your nearest autoparts supplier), using a brass suede shoe brush (NOT BRASS PLATED STEEL BRISTLES!), then a stiff toothbrush.
Once you have 'clean' plates, you can then make all the checks previously mentioned in this posting, again! Are the steel plates flat? Are the fibre plates flat? are there any burrs on the steel plates or fibre plates driving tangs? If so remove them with a fine file. Do you have a stiff clutch basket?
May I also ask if the clutch has been sitting in oil for a long time (greater than 12 months)? If this is the case the fibre plates may swell and you will not wash all the oil out and the swelling may remain.
What viscosity is the oil? If it is a straight SAE 40 W mineral, then this can be difficult to remove from the fibre plates.
Is the oil a mineral or caster based oil? If it is old castor oil, you'll need to strip the motor to de-clag the whole shebang!
Are the components (rods, roller and ball) of the clutch operating mechanism all in good order? Wear here will reduce the clutch lift.
If all your steel plates are in good condition and not warped, and the clutch operating mechanism is OK, then a new set of fibre plates could be the best option.
All the best, Bill