Problem solved.

I ran a new earth strap big enough to jump-start the space shuttle directly from the top fixing bolt of the starter motor to the battery negative post.
Volts are back to 14 on the dash meter, no more battery light on, easy to crank and start when leaving the gas station.

There's a bad earth cable somewhere (maybe more than one) not carrying enough load... bad contact, degraded copper, whatever.

After looking at the alternator reman process here and looking through the electrical FSM, (cheers Monty), I really didn't want to get into stripping out the aircon unit and messing with a "new" alternator.

Lucked out - worth a try if you have the same problem.

Cheers

Rob