Time to get back to basics Glenn. Courtesy of French Mike you can check all the ABS sensors and magnetic reluctors in the wheel bearings using a voltmeter.
The signal voltage changes from 1.7V to 0.7V as each magnetic segment passes the sensor, so jack each wheel in turn and slowly rotate it marking the switch points on the tyre wall with chalk. You should end up with 48 chalk marks looking something like this:
If any gaps are in the same place each revolution then likely a damaged reluctor, and if in random places suspect the sensor.
Other common failure causes are cable damage to the sensors, flakes of corrosion interfering with the sensing, or corroded plug pins at the ABS modulator. This was the rear wheel, and you can see the sensor was missing more than a few segments:
You connect the positive probe of the multimeter to the signal wire, and the negative to chassis earth:
I made up a cable from some old bell wire and a scrap ABS plug and socket to save pinning through the cable insulation. Here it is testing a hub prior to fitting:
