[PerryGunn]

Success - I’ve found the function that controls the tailgate opening only when the car is unlocked

The clue was in the HO-Wort - ENKF (Unlock boot vehicle)

Using the NCD/CAFD Tool, I found that this affected two functions in the CAS module
CLM 3002 > ENTRIEGELN_OEFFNUNG_HECKKLAPPE = nicht_aktiv [Default]
CLM 3002 > HK_GESPERRT_IN_GESICHERT = nicht_aktiv [Default]

Thanks to VikingOne, I was able to compare my car with one that allowed the fob to open the tailgate without first unlocking the car

The key function was HK_GESPERRT_IN_GESICHERT which was set to active on my car
The comment for this is ‘Entriegln der HKL nur wenn Fahrzeug Entriegelt (alt England-Logic)’ - assuming HKL is an abbreviation of heckklappe (tailgate) this translates as ‘Unlock the tailgate only if the vehicle is unlocked’ - which is a fairly obvious pointer...

Setting HK_GESPERRT_IN_GESICHERT to nicht_aktiv restores the old behaviour and I can now open the tailgate from the fob without first unlocking the car

NB I also had to remove HO-Wort OENE which is not normally present but had been added by BMW when they were trying to remedy the tailgate issue