I said GET home as soon and as carefully as you can, if the altinator light is on after flapping theres a good chance the Aux belt has gone!
She got home safely, so I went this evening armed with a new belt (6pk 1750 by the way) ..
Jacked the front up slightly, removed the wheel arch liner, and not only did I find a shred of belt jammed between the altinator pully and body, which I manged to remove, I ALSO found the following under the car:

YES, thats the aux belt on the drive shaft!!!
So after half an hour unwrapping the belt, I fitted the new one, tensioned it up, and ran the engine, at a fast idle (3k) for 3 mins, then re checked he belt, all looked ok..
Not sure why it happened, though I cant remember for the life of me if I ever changed the older oil saturated belt or not.
Under the bonnet though, being a late car its fitted with the forward cover over the radiator, I found there was bits of belt al over the place as it disintigarted...

Anyway, ALL pullys in line, and the upper plastic tensioner looked ok and inline too.
Told them to keep the forward cover off for a while to keep an eye on the belt on the HP Pump...