For Nige63 and anyone else who may be interested, here is how I installed the switch for my Thorspark ignition. The Thorspark is a cure for a knackered magneto but needs 6 volts (in my case) to work; uses just 0.25 amps, they reckon.
The idea was to make sure I couldn't leave my lights on by mistake, so I just bought a simple off/on handlebar switch (the usual sort) which now both turns off the lights (if I forget) and the ignition. It's also not obvious that it's the ignition switch so has a minor anti-theft advantage. The alternative was to have the switch near the battery but that meant running the wire to the lights all the way there and back again.
There wasn't much room inside the switch, so I joined the wire that feed the lights and the (green) wire that feeds the Thorspark close to the end connectors, inside the headlight. The fuse shown (supplied with the kit) is back under the seat.
Thanks Rog1.