I'd suggest try Rogers idea 1st, then go back to basics and trace every wire and connector on your loom to make sure nothing has come adrift or corroded. Your starter motor could be at fault or just the connections to it.
Yes I'd go to plan D and check the voltage regulator, not sure how to check it though.
You could buy a new one if not too dear too eliminate that.
Other than that try Kingys option lol
Make sure the cable from the battery negative to the frame is tight and clean.
Make sure the cable from positive to start relay is good and likewise the cable from relay to starter.
Make sure the starter is tight.
If it shows 13v when running it's charging, although 14v is ideal.
If there was a short you'd be popping fuses.
Might be worth putting the battery on charge then retest.
Still going through wires and chasing them through the insulation tape stuff. Being ill is not conducive to working hard in a cold garage thanks for the suggestions guys, will carry on checking them
I think I fixed it at last on Saturday. For now anyway.... Didn't wanna post till I'd ridden to and from work to check how it handled but it did perfectly.
From investigations, swearing and headaches, it was the battery. Though giving off the correct voltage, it didn't actually have enough and the whole thing was dead. It'd been dying for some time, but the recent weather sped up the dying process.
Since replacing the battery, the electric start has started working again (a sign to me that it has been dying for a while, the electric start stopped working in September).
Thanks for the help guys. Including Kingy, who only gave me thoughts to warm me up