look at your rear sensors mate, i just did mine today after driving for a year with the problem
engt
January 2011
my abs light on problem, was grease build up on a rear wheel rotateing magnetic ring, which collects metal particles off the brake pads,and creates a problem with the magnetic field and the sensor, cleaning the grease off the magnet and sensor fixed my problem, this was a 2003 1.4hdi just out of interest, hope this helps.
keith
November 2010
put car onto a code reader for faults logged, as its telling you that there could be something wrong with your abs system.