That's nice. Did you get home once the mag cooled down ?
Called the AA - and guess what? They arrived about an hour and a half later and it started, despite me having tried it every 10 minutes up to that point. Chap had a folding trailer in the back of the van so he gave me a ride home rather than me risking doing it in several stages.
Now all running fine after a condensectomy and the fitting of a Brightspark condensor. Although I have since fitted a fully rebuilt spare magneto. Otherwise it's a very pleasant ride. The engine is, I think, on the standard 6:1 pistons and it pulls very well since I put it on solo gearing.
It looks better in the photo than in real life. It's an un-restored survivor having obviously been maintained on a budget (remould quality tyres when I got it) and having had a chair on it for decades. I like them like that - there are plenty of shiny bikes about.