Have had the car over at Thrussington Garage today getting new Kumho PS71 boots put on all round (after finding a few weeks it had run-flats fitted at the rear when I bought it…).
Mike spent some time having a look at the electrics for me, and agreed that the wiring itself is all good from the door switches, through the rear panelling and into the loom that traverses the transmission tunnel to the front of the centre console (pointing out that if there was a short the light would be on all the time, which seems fairly obvious now I think about it!). Unfortunately that was all we really had time for.
Next step will be to get the full wiring diagram for the car (which seems to be about as common as a fish with legs) and trace all the components. It looks like it could be a faulty contact with the Body Control Module, or a failure of that itself.
Onward and upward!