Thanks for the detailed reply Rally Matt.
In response...
Correct gear oil was used.
Flywheel issues certainly could be the cause of the judder, but as I'm pretty sure the clutch is fully disengaging when the pedal is depressed, I don't think this is related to the difficulty getting into 1st when hot.
Worn location hole - Can't prove either way, but i'd consider my driving style one that doesn't cause excessive wear on clutch/box components, even on my race car I've never damaged a clutch or box, bushes are the things I go through a lot of! However, clutch judder could be a contributory factor here so it remains a possibility.
Worn gear linkage - Although not as tight as it was when new, I'd rule this one out because gear changes are smooth and precise when the car is cold and when the engine is switched off.
Cover fingers - Could be worn, but again, I'd expect some kind of drag when changing gear. The issue is only with first gear. If cover fingers were worn/damaged then I'd expect some kind of tell tale sign with other gears at certain speeds, and there are none. i.e, it wont slot into first from stand still when car is very hot, but it will slot straight into 2nd. Cover fingers may be a contributory factor to the judder though.
So, based on your own detailed response, I think again, the finger is pointing towards synchro issues.
I may try an alternative gear box oil before considering changing the box. Not sure what yet, I'd need to read up on it and find something that may improve the problem.
When I eventually come to changing the clutch (if I ever do), I'll probably get a recon box and switch the box at the same time. That will most probably cure the problem, but unfortunately it will never give absolute certainty as to whether the problem was actually with the gearbox or the clutch. It does seem likely that we have synchromesh issues here though.
So, the next question to help confirm that, is a worn synchromesh more apparent at higher temperatures?
If temperature makes absolutely no difference to the function of a synchro, that leaves more questions unanswered, if it does, then I'm pretty sure we've established the most likely cause.