update; hopefully it benefits people with the same problem:
So, with a working cable in hand, i've managed to dig deeper and find out exactly what the whole deal is with this transmission fault. i'm almost convinced it's an issue to do with software rather than hardware...
INPA shows error in the EGS module code 4F8D, quick google redirects to a post on e90:
http://www.e90post.com/forums/showthread.php?t=1036295
I have always been very skeptical of all these finicky process driven solutions, but I understood the logic behind what the method was trying to achieve, it was obvious that the aim was to change the adaptive values stored in the transmission ecu through rote repetition.
But what was most valuable from that thread was figuring out how to successfully recreate the problem over and over again. I manage to have the transmission fault pop up at command knowing what driving patterns were bound to give me the error. From the thread, it was the 'D' downshift pattern (or engine braking with no interference from using my brakes) from gear 5-4-3 to stationery. So I tested that on a quiet street and sure enough, I had the fault come up each and every time it slowed to a crawl using engine brakes.
The fix? I would've assumed the workshop would've done this during service as I was told it was during the handover but I didn't wanna leave it up to chance and did it with INPA again anyway. I reset the adaptive values in the transmission menu and verified the mappings were all showing 0. Drove my car out and attempted to recreate the transmission fault again, voila, this time car was still slightly jerky but no error came up. Did this for another 10 or so times and I'm now fairly confident the car has learnt the proper downshift and slip differentials in the ECU are in tune. So the problem seems to have been solved, for now anyway.
If you don't see any further updates from me on this thread, then the above could be a helpful and inexpensive fix if your mechanics are unsure why your transmission is giving you errors and are suggesting a rebuild to make it go away.