I had plug fouling issues for the first 2 years or so, always having to carry a clean spare in the box. Eventually, it started fouling up in town, having to be wheeled to the side of the road and fettled before it would start again. Tried a new HT lead, and finally resolved it by replacing the ancient Bosch coil, which was clearly giving up the ghost when warm. Never touched the plug since.
Coil was one for a BL Mini out of Halfords.