Thanks for the help guys. That Bomar flying service link was a godsend.
The dead short to earth was a red herring brought on by my crap multimeter. These new digital ones really do give false readings as they have hardly no internal resistance. I wish I could have an old AVO like we used to use in the airforce!
After replacing the the CB, alternator and fixing some bad wiring he was good to go.
Cheers