Quick update on this clutch saga, after a big bag of parts from Fowlers turned up:
Replaced push rod (and oil seal by sprocket). Old one was corroded by sprocket which appeared may have hindered movement and had a dimples worn either end.. Seemed to help a bit, but still a pain to get into neutral when hot.
Next I replaced the steel clutch plates (with some new TRW ones) and thrust rod in clutch basket. TBH thrust rod wasn't too bad. Old steel plates were very blue in places, so probably should have been replaced anyway. Whilst there, I inspected in more detail the clutch basket for notching by the old steel plates - there was a little but not too bad. I gave it a light file down anyway to make it nice and smooth. However, after all that the clutch sticking was worse! Couldn't get it into neutral at all, not even when cold!
I then swapped the EBC heavy duty springs for some brand new genuine Yamaha springs. Clutch much, much lighter, no drag at all, slides in an out of gear and neutral super easily now. Even when riding slow round London, or out all day. However, it now slips a spot when I really accelerate hard... :-/
So, my suspicions are:-
New plates are thicker and the heavy springs are just too strong to release them enough - hence the drag.
Genuine springs are much lighter, so happily release the plates enough, but are too light - hence the slip.
My next plan is to try fitting half Genuine Springs and half EBC, 3 of each, offset evenly, to hopefully achieve a happy medium. Anyone else done this, or foresee any issues with this plan?