But the total cost has now come to £1060
It turned out to be the N/S rear phonic wheel giving intermittent reading. The alfa garage tech specialist connected his laptop so that he could scope out the ECU signal from inside the car whilst travelling at 80 mph. He had done this for the front two wheels in the garage however he had to do this from the car for the rears.
This was the last chance for the car, I had incurred the following costs up to this point:
1. Both front speed sensors - £200 - Don't change these unless the ECU error code says change them
2. Scoping of front wheels - £60
3. Both O/S wheel bearings (include the phonic wheels and the rears have the speed senor integrated) - £500 - done on a 'hunch' by the garage
4. ECU test - £50
Once they scoped out the rear wheels (see attached images) and found the problem (see the dip in the middle of the N/SRear image) I had to pay £250 for the N/S wheel bearing. Fortunately changing this wheel bearing also cured the ABS kicking in too early when breaking hard.
Along the way I have gone through disconnecting the front speed sensors in turn and seeing if the handbrake warning light came on in addition to the ABS warning light (this happens when the second error is registered by the ECU). Note that disconnecting the front O/S speed sensor meant that a second error was not registered - it must be linked to the N/S rear some way. As a way of working around the car excluding each wheel in turn to find the problem wheel I had also tried connecting one speed sensor to both ECU inputs however the ECU spotted this.
I hope this post helps anyone else with this error to determine the problem without having to spend a grand like me!!!