2 things to check, I suspect the reluctor ring has a tiny hairline crack in it and is moving on the drive shaft ! you will need to inspect the detail of the ring, when they crack its hairline. If you can move it around on the drive shaft then its knackered ! There is a company that make replacements without buying a shaft.

2nd thing to check is the tyres, Is one different make ? and check the sizes as well. If its a different make you need to check the overall circumference compared with the other tyres. It is suprising sometimes how different make can vary enough to trigger off the ABS after a few miles of driving !

Hi everyone who may be interested in the outcome.I decided to have a closer look at the rear sensors for anything obvious,despite obtaining some acceptable resistance readings.I took out the offside sensor which came out quite easy,which having read some tales of having to drill them out,was a good start.The sensor tip had rusted over,so with some fine emery paper I managed to get back to shiny metal without taking too much away.I replaced everything and the light still came on but less often,so some improvement had been made.So over to the other side.Whilst taking out the sensor I realised someone in the past had made a repair to the cabling.I removed the black insulation tape and lo and behold one of the connections had loosened(which probably explains why when loaded the light would go out,maybe just making a contact in some way?).After reconnecting all the joints properly and again taking the rust off this sensors tip,I took it for a 15 min. run with an empty boot,and what joy.No more ABS light on!