Bit late getting back this. Looks like the project temporarily on hold, no funds, but many thanks for all the replies.
I think the Comet starting system was the model for this conversion. They are having to run with 6 batteries due to the starter gear ratios, which dictate a higher starter motor speed.
The Hunter conversion looks particualrly interesting though, especially the bit about the changeover switch, which must have been real fun to wire up. When this project does go ahead, will probably build a constant current charger to charge all the batteries in series, running from the gpu...
Chris