tommyardin good shout on the bolt cleaning,but don't put vaseline (it's not conductive) or anything else onto bolt or area before refitting bolt put it on after, same goes for all connections like this including battery terminals, their needs to be metal to metal for a proper contact.
Ive got a little tub of NO-OX-ID grease which is specifically intended for use with electrical contacts.
I took a look at the connectors under the left panel at the front. They look really really good IMO (and I do a reasonable amount of work with electronics etc), still very shiny and golden/brass in appearance, no typical green corosion. Perhaps a little discolouration on the contacts for a thicker black wire which is probably the ground. I did measure its resistance and it came out at practically zero ohms (0.2) so probably that connetion is ok.
The bike starts just fine, so I dont think there is anything funny going on with the connection from the battery to the chassis. I gave it a quick look and it seems fine - not loose, no visible signs of anything untoward.
I added some NO-OX-ID to the connector under the fairing and pulled the connectors apart and pushed them back together a couple of times for good measure. Of course, everything is behaving right now, and I suspect that if I need to dig any deeper Im going to have to take the dash off completely, and right now the cold weather doesnt lend itself nicely to a long project like that.
I also measured the voltage at the battery terminals with the engine off, on, and with some revs. It was varying anywhere from about 12.9 up to about 14 volts, so I suppose the alternator might also be fine and is still able to supply charge to the battery and other accessories.
Yet to take the petrol tank off and look at the connectors under there. I might try that tomorrow. For now the mystery remains...