I spoke too soon. The light stayed off till 40+ mph then came on and stayed on. Stopped the car for a while and on restarting the light was on from the outset. Switched off and back on - still the same.

Seems I have a loose wire or connection somewhere in that area, but the oil light is working again! And now the rain is on again.

Thought I had some spare ABS sensors on the shelf, but no idea which is which or if they are any good. Beginning to wish I'd just stuck a good old-fashioned servo on this thing!