OK I will bring an end to this thread to help anyone in the future.
After some messing I managed to fit the new ECU into the car, bizarrely there are actually three bolts holding the ecu in (rave says 2!) and the top bolt is a proper PITA. Moment of truth when putting the key in ignition and result NO ABS FAULT with the ignition on.. Joy... short lived "Traction Failure" as the pump built up to pressure... Ah. Not to worry I thought we will clear any error codes that might be there (could have been some in the replacement ECU etc) we cleared that (it did have some codes about front sensors) and no error codes left - fantastic. But still no check bulbs on the dash - looks like they have been removed then, another day another job!
Sadly I then ran out of time to actually drive the car.
A few more days later and we've been out in the old girl today and although no ABS fault with the ignition first on we still had Traction Failure as the pump built up full pressure. Anyway after driving a few hundred meters we then got ABS fault... (I know most people would cringe at this but at least its waiting to report a fault until after something has happened!) so I took it down to the garage got the snap on code reader back on and we had two errors for the rear left sensor (again!) Output low and intermittent. However, the sensor I put on it in the first place was a cheap ebay job. I retained the original wabco sensor as it was dated 2011 (how long the car appears to have had this fault) so quicly whipped the wheel of and popped the sensor in. Bolted her all back together and took her for a little run around the block. Still Traction Fault after the pump had built up to pressure but not a sniff of ABS FAULT anywhere (hallelujah!) back into the garage and back on the code reader and cleared the original faults. In hind sight I perhaps might have saved a step if I had cleared the fault first but I was more keen to see if ABS FAULT immediately after the ignition was turned on came back on.
Anyway, with all faults cleared and its original sensor back in but still the replacement ecu no ABS Fault after driving or on igition and most importantly now no traction failure either!!!!
Some morals here:
Dont buy cheap sensors
You need detail diagnostics (I expect cheaper alternatives would have reported rear left sensor fault in all situations and caused people to think that actually their original ECU was OK when infact the new sensor was to blame the second time around!)
Final thoughts? Well I wouldn't be overly surprised if the root cause of all this perhaps 5 years ago a sensor was damaged which caused damage to the ABS ECU, the offending sensor was replaced as per diagnostics but it didn't fix the problem. The PO then elected to remove the ABS and TC bulbs from the cluster to hide the fact that the car had issues and its managed to get through an MOT with an improper test and thats the way shes been for the last 5 years. Until muggins here got involved "oh I'll buy an expensive P38 it will be less trouble that way..." yeah... right!