Well after disecting my car, it looks like the adaptor actually connects into the wiring loom in the back of the external display (clock/temperature/radio channel).
To get at it I had to remove the steering column shroud, then the A-pillars (clipped on) and then the top of the daskboard fascia which was fastened with Torx screws in 7 places - three along the top one each side and two below the instrument panel.
Then I fiddled about trying to fit all the wiring in behind the stereo and get the stereo back in place.
After all that, I discovered the ignition and 12V wires were transposed on the adaptor so when you turned the ignition off it asked you for the security pin when you switched it back on. I spoke to the supplier and he said that this is normal - you can swap the two wires back around again on the stereo wire inputs.
All in all - a very fiddly job!
Thanks for your advice.
Ian.