Steve,
A similar thing happened to me, on my WM20. It would have been around 1984, on my way to Portsmouth for a show. Stopped at Wisborough Green, for a break, and went to start it, no spark. After a lot of fiddling I decided to call the RAC recovery. They were quite busy and not able to reach me for an hour or so. After a while, I tried again and the bike started, so I had to huriedly call the RAC back, they were good about it and said to call again if I had further trouble. Carried on to the show, with no more trouble. The return journey was non-stop all the way .......... and the magneto was changed, never had a problem after that.
regards, Richard