The only strange thing was that the carbon on the piston tops was breaking up rapidly so I wonder if the fuels I was using was washing the carbon off and dislodged enough to foul a plug and stick a valve? I'm going to put a new set of rings on, re grind the valves and build it back up and see how it goes, it was starting to use oil so it could do with rings and a hone anyway. Good thing is its not going to cost me a fortune, bad thing is I still can't tell what brought it to a halt. That's life, it was a good excuse to look at the bottom end anyway and say hello to the camshaft! Another day in the garage but hopefully get out for a run tomorrow...
(not on that particular Norton)