Hiya John-SF,
I've come along in your 'hiatus', good to see your enthusiasm and progress, I started building mine from a frame and minimal ancillaries, that I found at the local wrecker back in ~'77, had it rego'd (your MOT) a couple of times in the '80s, and started it briefly ~ 8 ago and last rode it about 12-14 yrs ago.....(I can't believe I just said that!!)... so that's a kick in the duds for me..!
About that 'heavy' clutch, I read about a similar thing recently, and got thinking later that maybe it could just be the push-rod through the mainshaft being a bit gungy(notice that end with a 'y'), you can pull it out easy by taking the oval inspection cover off the gearbox, then the rod adjuster locknut and screw, and pull the rod out- a bit fiddly maybe at first, but you'll work it out? (if it doesn't come easy, a indication of problem, if easy, I could be wrong!!)
If it's stiff give a squirt of penetrine type stuff up the hole, or better put something under the mainshaft nut to funnel stuff out, lean the bike to RHS, and give the shaft orifice a good reaming with solvent, then pump a bit of lube in(ideas on that?) and it should slide nice...?
As I said I could be wrong, but you can't do harm 'specially after sitting so long .
Cheers good luck with it cheers, duTch