The on-going saga of the difficult gear-changing M20 gearbox (actually it was an Empire Star box) has reached a successful conclusion.
The problem was in the 1st-N-2nd area mainly, and although 3rd and 4th could be woolly they weren't the main problem area.
The box was stripped and cleaned then the end-floats were adjusted both left, right and centred all to no avail; I managed to get the box to go N,2 3 4 acceptably but still it would not click nicely into 1st. When "helped" through the trap-door the dog would easily engage with the 1st gear layshaft pinion, but not by the selector fork action alone.
The conclusion was that this pinion (due to 70+ years of wear and crashing into gear etc) needed to be further "on" the shaft to allow the slightly worn dogs to sufficiently engage. I therefore (more in hope than expectation that it wouldn't be too hard to machine) mounted it in the lathe and with a sharp tool took 30 thou off the shaft bearing/wearing central ring area. Result...a gearchange as positive as you could wish for. Not quite AMC standard, but more than acceptable.
So special thanks to Ian Wright and the others for contributions and suggestions.