Well slap Bubba's shrimp and gimme a ping pong bat...
I made it!
27.5 miles of M2, sidelights only and no other electrical drain.
The battery light was like a flickering candle, unless you approached 3k revs, then it was the bright red glaring back at me!
Before I turned the engine off I tested it again - 11.2V at idle, and 11.9 at 6k rpm. So, it does have some charge effect, but looks like the alternator is gonna need swapping...
I will phone around for a second hand one in the morning, all suggestions welcomed!