Pulled over and it died. Got a jump from some friendly people and was on my merry way.
Then the same thing happens, 30 min walk from home. I was thinking battery at this point, but also possibly the alternator not generating anything.
So I walk home, swearing a lot, and pick up the AW11 which has a new battery in it. I drive it back to the SW20 and put it in that.
Crank it over and comes up fine.
One thing I noticed is that the volt meter would rise when I revved it.
I got back and noticed Mr Revhead was on so started pestering him (And if you read this, the cheque isn't going through, reported non-existant domain name. sw20srule.aw11suck.com works though

He suggested that it could be the voltage regulator on the alternator.
He mentioned that he's not that great with sparky stuff, so I'll take his word for it.
However, is there anyone here who can help me clarify where the problem lies?