Well, quick update, the crimp pins arrived yesterday, so I couldn't wait and the sun was shining for a change.
I followed the excellent write up by Dougie175 here :-
The BCM on a 2015 UK RHD car is located behind the fuse box in the drivers side footwell, to get access to it, remove the two torx screw that hold in the fuse box, the fuse box will then drop down and back giving just enough room to get into the BCM.
The connections I used were exactly the same as in the post I linked to above, that is T52A pin 32 for the switch and T52B pin14 for the LED.
I crimped two pins onto some lengths of thin equipment wire, and disconnected the vehicle battery, I then removed T52A, and has been posted before, the crimp pin will not go in unless you split the connector.
Remove the 'shell' that covers the wiring, there are two clips on the end that need a small amount of pressure and the shell slides off in the opposite direction to the wiring exit.
Then, the two halves of the connector slide apart, it is dead easy then to push the crimp pin in, they only go in one way around, there isn't much room to get your hands and a torch in the space, but it isn't too difficult even for my aging eyes.
Once done, slide the two parts of the connector back together, then slide the shell back on.
Repeat the procedure for the other BCM connector, the crimp goes into T52B pin 14 which is the feed to the LED behind the 'off road' switch.
Then, before putting everything back together, I ran the two wires back to the switch panel in the center console, cable tying it at various points to stop it falling or catching anything that moves.
I reconnected the battery, and was greeted by several warning lights, TC, steering, tyre pressure etc.
Running VCDS cleared most of them, except the TPMS and steering, the steering one cleared itself after turning the steering wheel fully left and then fully right (my mate who used to work at a VM garage says he's seen that many times and is normal?)
Whilst I had VCDS open, I did the coding, which was exactly the same as described in the link above by Dougie175, mine also didn't show 'bit 2' so I had to change the byte from 40 to 44, on the ABS module, my value was different from Dougie175's so I simply added 512 to the value shown and that seems to have worked fine.
As the switches have not yet arrived, I measured the voltage on the wires, there was 12v on the switch wire, and 0v on the LED wire, as a test, I connected the switch wire to 0v and the symbol on the dash (green car going downhill) came on, the LED wire went to +12v, shorting it to 0v it again the symbol went off and the LED wire went back to 0v, so everything looked to be working
I then re-assembled everything and soldered a small pushbutton momentary switch to the switch wire and the other side of the switch to 0v so I could activate it whilst driving and went for a test run. There is a really steep hill about 1/4 a mile from my home, so that would be an ideal test location.
Once I had started moving, after about 300 yards, the TMPS light went out, I guess it needed to see some pulses from the wheel sensors to clear it, I got to the top of the hill and started going down at about 20MPH, I hit the button, the indicator came on on the cluster and the brakes started to slow down the car to a walking pace.
I also noticed that the acceleration has changed massively when in 'off road' mode, you really have to press the pedal down to get moving, pressing my switch again to take it out of off-road and everything returns to normal.
So, a very successful modification, which should be really useful when we go to the Scottish Highlands over winter.
I just need to sort the Haldex pump error and await the proper switch panel arriving now.
Running VCDS now shows no errors other than the Haldex one after a test drive, so everything is good.
So thank you to everyone who replied to my original post and most importantly thanks to Dougie175 for his excellent post.