I've very likely just found my problem.
Valve lash wasn't that far off. I had previously measured at the largest available gap (going by sight), rather than at TDC, according to the "TC" mark on the flywheel, and the row of marks on the housing.
So, I set the valve lash, and just for grins, thought I would check when the points were opening, since the timing marks are now right there in plain sight.
I think I just found the problem...
The points are only opening once every two revolutions of the crankshaft, and when the points do open, it is at about 120 degress past TDC (where the TC mark on the flywheel lines up with the 0 mark on the engine case.)
The 120 degress is definately wrong, the once per two revolutions is definately wrong.
What could be causing this, and how serious is it?