It ran fine when I was setting the points, but then wouldn't start when I put all the covers on and tried to go for a ride.
No spark at the plug. Remove points cover. Check points, Big fat spark. Refit points cover. No spark at plug. Remove points cover. Etc.
Round and round in circles I went, for several hours, until I realised that I'd left the electrical connector to the points standing ever so slightly proud, and as the cover screws were tightened up it was shorting to earth.
I only discovered this because I'd reached the point where I'd resorted to replacing the points cover while the engine was ticking over, to try and find out what was wrong. I must have reset the timing and re-gapped the points about twenty times before trying this....