AF1:
… also check for corrosion Iin the lamps themselves.
Yup, that was it.
There was a crack in the lens over the lamp near the driver’s side step and evidently it had let a bit of water in. The lamp was working, but I guess not at full brightness and enough to cause the sensing circuit to notice. I cleaned up the bulb and the holder, repaired the cracked lens with perspex cement and everything is hunky-dory. Passed the MOT this afternoon.
Thanks for all the help guys! I hope not to have to trouble you for another good long while. 