Thanks guys,
It didn't do it when I left work this evening, nor when I stopped partway home to pick up some bits I needed, so it looks like yet another intermittent electrical problem
I'm getting very tempted to rewire the whole bike, but I don't have the time as I need it for transport.
Oh well, looks like rebuilding my Doodlebug (the SV400) is highest priority, then I can use that and sort out the 650 properly
Doof