Well, after fitting my grips, and giving the bike a wash (I may have missed a bit... you'll see later on), plus some anti-corrosive, and giving a final once over, took the bike for it's MOT... However, I overlooked the number plate light, probably because it was daytime. It wasn't working, so it failed.
Checked the wiring behind the mud guard, and voila. It's a proper faff to get to, especially if you've got pannier racks on, but my positive reg plate light wire had chafed through in two spots and had corrosion a good way up both directions. A very gentle pull broke the wire on both sides of the connector, and the connector was practically welded shut with corrosion on the pins.
Fixed it (had to opt for some bullet connectors as the connector wasn't salvageable), bit of convoluted tubing to stop it rubbing on anything else, and some silicone grease in the other connectors/bullet connectors to keep the water out. The indicators were OK, no corrosion of any kind on the pins and the wiring was also in good nick.