At first I thought I couldn't engage neutral. Then it became apparent that it was actually the neutral light that was at fault. Further investigation relieved that I had also lost indicators, fuel light, headlights, horn, brake light and probably a host of other things as well. I could however start the engine on the button. Seeing as I was in a lay by in The Loire, this was not great timing!
Two fuses had blown, and after replacing them, a few things started working again, but I only had one front right indicator, and no electrics where working at the rear of the bike.
Now, I had read about the loom under the back mudguard rubbing on the tyre if you have a heavy load, and I was certainly loaded to the max, but mine seemed well routed, close to the edge all the way around. What wasn't immediately apparent, was that there was a loop in the cabling as it entered the rear cluster, and this was where the root of the problems lay. The tyre had worn through the protective sheath, and all but one of the six or seven wires within.
Fixing the problem was fairly straightforward:
Undo the 4 nuts holding the tail light, and number plate to the rear cluster. No need to undo the bolt and nut inside the mudguard(ask me how I know this).
Unplug the three connectors.
Bend back the clips holding the loom under the mudguard. It's tight under there, but this is possible with a screwdriver. They are only wielded on one side).
The loom in now easily accessible.
Splice the wires back together(there is enough slack to do this, and in fact the excess slack is part of the problem), and protect with plenty of PVC tape.
Refit the loom, making sure it is fixed in such a way that it cannot rub on the rear tyre, particularly when it routes through the hole into the rear light cluster. I used a big cable tie inside the light cluster to prevent the loom from sliding out into harms way.
Bolt everything back together.
Finally. Gave up on camping for the day, phone ahead to Brad and Karlia at bikerskrib.com and arrange for some outstanding hospitality and a comfortable bed for the night. Just a 300 mile ride away! Thanks guys, you are totally awesome!