My guess is that the battery issue is down to the BMW electrical system/canbus etc. It appears to be doctored a little to suit the Morgan and so has a lot of unused circuits like traction control etc.
Perhaps it creates a drain somewhere that is hard to identify. If ever you leave your key in the ignition (turned off) by mistake, within half hour the battery will be drained enough to prevent starting. I guess that might be down to some kind of immobiliser as well.
Whatever it is, it can be very frustrating!
