Time for an update...and a big thank you to everyone who has gotten involved.
I fitted the '7 new plates' but that didn't work. There was no clutch movement at all. To confirm that it wasn't my lack of mechanical 'skills' I fitted the old stack of 8 plates after cleaning them again - and they worked perfectly.
I contacted Bob Wrights and had a good chat with him about bikes, trackdays, contorting old fat bodies to fit on small fit bikes etc.
He confirmed that the 'new' plates were not up to scratch and told me to throw them in the bin. He sent me a new set of Surflex plates. They weighed in with a 2.9mm width, as opposed to the other plates which were a full 1mm thicker.
I've soaked them in oil and I fitted them to the bike this morning. All seems to be good. There's a bit of 'stiction', but nothing like the old plates - so I think my problem is fixed, and I'll keep the old ones for a set of spares.
The sun was out today, and spring appears to have sprung, so in a rush of blood I've gotten a bit carried away and the bike is now being completely stripped. I want to rebuild and grease the rear suspension, and fix up a number of other little things (brakes, new piston rings, new decals, maybe even repainting the poor thing).
So I won't be able to confirm anything for a few more months - but for now I think that the clutch issue has been resolved.
Take care and keep safe everyone. I can't wait for the time that we can actually get out and enjoy our toys again.