UPDATE: Got the car diagnosed at a friendly local BMW indy rather than a stealer. Found just the one soft-code of 'clutch-slip, sporadic'.
Deleted the codes and test drove, only to find the same code returning.

So the car has just come back from the transmission specialists who have rebuilt the torque convertor, changed the fluid (and filter at my request) and re-fit it.
The mechanic said that I was lucky that I nipped it in the bud and got it done straight away. He said only the torque convertor seals had gone and the internal linings hadnt started to come away yet. Therefore the gearbox itself is in great shape, with no nasty filings e.t.c in the sump pan.

I've only done about 20 miles in the car up to now, but initial impressions are good so hopefully that has done the trick.

Time get my hands dirty now and tackle those damn valve cover gaskets!

-Ben