Hmmm. New problem. It stopped running on the lower cylinder after I accidentally left the fuel petcock on overnight. Plug was black and rather covered in fuel. Plug now doesn't spark even against the head but was running on lower cylinder before. Fingers crossed it'll go properly in the morning with a new plug and without the lower cylinder being full of fuel!

I think I might have fouled the plug in the first place by riding like a nancy. I tend to tick along at 5k but by the sounds of it it'll run better if I drop down a gear and fang it?