More info.

I went thru all the stages in Jon@work's MAIN ELECTRIC VALVE TESTS and CONTROL ELECTRO VALVE TESTS and got readings within parameters for nearly everything. The exceptions were:
Connect ohmmeter between ECU connector terminals 11 and 18 - should be 2-5ohms, I got 1.2
Connect ohmmeter between ECU connector terminals 1 and 11 - should be less than 1ohm, I got 1.5

Also started on the WHEEL SENSOR TESTS but I was on my own so could only really check resistance. At the ECU connector and they all came in between 1048 (LH Front) and 1065 (RH Front). The two rears were both 1060.

I put a spare ABS ECU in and the light went away and the oil pressure light came on, but as soon as the car hit 45-ish the light came on and stayed on after the car had been switched off.

I pulled it out again, reconnected and drove the same route at 40mph so keeping the light off. Came home, parked up and restarted - no light.

Refitted the original ABS ECU, went back out and as soon as I hit 45 the light comes back and stays till I disconnect again!

So, for the MOT I just won't drive fast on my way there, but any more ideas?