Went to the docs this morning to find out about the weirdness with my left arm. As I suspected, it seems that the ulnar nerve is trapped/pinched/compressed, probably at the Cubital Tunnel in the elbow.
So, what does this mean? For starters I'll have to put painting on hold for a week or two as I need to rest that arm and try to avoid bending it at the elbow. I also have to try keeping the arm straight while asleep by wrapping a towel around the elbow. If rest doesn't sort it out in a week or so, then it might need further investigation and possibly surgery to shift the nerve to the inside of the elbow (even more time away from painting minis).
Assuming that resting for a couple of weeks sorts it out, then I'll need to make some adjustments - I've already bought a keyboard wrist-rest which should provide a good cushion for my elbow when painting.
I guess I'll just have to base up my unpainted Sarmatians to use during my club FoG tournament as the chances are that they won't be finished before my first game.
In the meantime I may just have to do some actual wargaming ;-)