It turns out, not surprisingly, that reading the instructions would have been sufficient. There were two points I missed in the instructions for the car:
- I didn't have the ignition on to the second stage. This may not have been critical, since, as noted above, there was clearly some communication going on, but I did it anyway. Specifically: ‘Switch on the ignition, and, in addition, press the start/stop button without operating the brake or clutch pedal.’ To be honest, I had never done that before, and didn't even realise it was a feature.
- This is the critical bit. When entering the PIN using iDrive, it is not sufficient to enter the PIN and just wait, which I had been doing. About a dozen times. The label ‘Confirm passkey’ that appears on the screen during PIN entry isn't just a label, it's a menu choice. So the sequence is: enter the PIN, select down on the controller to highlight ‘Confirm passkey’, and then select that with the controller.
What I had been doing was getting to the final step, and then letting the whole process time out. Repeatedly.