There is back feed from the ignition light. I connected the relay coil to the ignition switch terminal which supplies the ignition light, leaving the kill switch functional so that I could check the relay. Started the bike, let it settle to a steady idle. Turned the ignition switch on, relay latched in. Turned the ignition switch off, relay stayed latched in. Checked the voltage on the live side of the ignition light ( same as at the ignition switch) and it showed around 3.5 volts at tick over. Disconnected the ignition light at the switch leaving the relay connected. Started the bike with the ignition on, relay latched in, switched ignition off relay latched out. Connected the magneto kill wire to the relay dispensing with the kill switch. Ignition off bike won't start, ignition on, bike starts, ignition off bike stops.
I'm not bright enough to work out why there is a back feed so I think I'll try reconnecting the ignition light to the switch and connect a diode in series with the bulb to stop the back feed but still allow the feed to the bulb from the switch.
Mac