Update, Cars now back up and running for Snetterton on Sunday with a repaired altonator.
Diodes were burnt out, more air feed to the back of the alternator has been suggested.
I have not done an amp meter draw test yet Jon but the car seems to be charging higher now on its fast tick over than all season, so I suspect its been dying for a while
Jon here's the diagrams I promised below.
p.s. did the roll bar e-mail get across to you.