To update this thread, I've had the battery off the bike for a while (a couple of days).
When I plugged the battery in, after checking all connections I could see, I turned the ignition on.
I was greeted with a backlit LCD, and nothing on it, the neutral light was on, but not the oil light (as it should of been, since the engine wasn't on). The rev counter also thought the bike was doing 7k RPM without me hitting the starter. Great!
So my first thought was fubar'd clocks. Expensive, and would reset the mileage of the bike. Gutted.
A little poke with a DMM... with the engine off, the battery was at 11v, a little low, but not that unsuprising considering it's been sat in my shed for a few days.
So I tried to start the engine to give the battery a wee charge. Fine, all good.
Then the lights started to fliker randomly, that's odd, what's worse, is that the rev needle is moving left/right - not overly, but definitely noticable (maybe 500 RPM either side of "centre"). That means the clocks are getting power OK, but that power is all over the shop.
DMM on the battery again, 0.4V, 9V, 12V, 4V, 15V... ah, I see now.
Unplug the reg/rec, DMM on battery and a constant 12V.
Conclusion, Reg/rec is fubar'd. They're cheaper than clocks. Hur-ray!
EDIT: I just picked up a reg/rec on ebay for £23.95 inc P&P.
Alternator output has been checked as well, and that's spot on the money. It looks like with all my gadgetry on & off the bike over the last 2 years, I've cooked the reg/rec.