And I had to take the BSA out for a plod around the countryside. I couldn't start it from hot when I first went out for a long run, and the capacitor seemed suspect; I bought a Thorspark ignition but needed to see if the Magdyno was actually working properly. So far it seems fine, and I had no problems starting it from hot, so it will be time to venture further away from home...
It's a really enjoyable ride, floating on the saddle while the bike works away underneath you, and I'm keeping a good look-out for potholes, or at least a smooth route between them, having been launched into the air a few times.
A very satisfying couple of days.