Thanks Ian, a good idea but it's a vacuum tap on the Classic. Perhaps the earliest air-cooled rotaries had a manual on/off tap, but not sure it is worth looking for one to swap. I'll be putting an additive in the fuel which I hope will help avoid future problems.
It started fine again today, but I notice the charging light isn't going out until quite high revs (2,500-3,000?), which doesn't seem right. Can't say I have monitored it in the past though. Battery is only about 18 months old, and put on a charger every now and again.
Unfortunately I understand even less about electrics than I do about carbs! Mike