Burned out computer module
Pulsey, Thanks for the great pics and guide. As you can see from my pics, I was too late. The ingress of commutator dust shows up red on the preamp board. Yours and many other posts have identified the same cause of failure.
The electronics are not adequately isolated from the motor unit. Although the exit holes for the power connections are quite small, they can clearly allow the metallic dust through. The lacquering is pretty ropey, If I prod about with a continuity tester I can easily make connections on the exposed tinned surfaces. Hopefully I can send the whole motor off for a rebuild. That is, unless anyone can supply the module alone. I will use the appropriate thread (not this one) to vent my frustration with BMW service.
It seems obvious that this pathology is an inevitable outcome of the flawed design, and yet customers are having to fork out £900 for a solution.
Sorry I started venting......