So... In case anyone else stumbles across this in years to come... I managed to code the new panel successfully and now it is all working as it should
I used NCS Expert to read the IHK module code from the old unit, then swapped them over and wrote the code back to the new one.
Comparing the code in more detail with NCS Dummy, the differences were many - the new unit was from a 4-cyl diesel 1-series, and my car is 6-cyl petrol 3-series, they have different water pump and heater setups, and many different parameter settings, so many differences in fact that I'm surprised it worked at all!