It could be sensors on the wheel about £40 each but the first thing to check is the 48 teeth rotors on the CV joint which have a tendancy to rust and fall off. Citroen will want to sell you an entire driveshaft for £207 +VAT but Neat Automotive will sell you new rings for £20 each and these can be put on fairly easily just by detaching the hub from the CV joint.
If your brakes are working well and you don't care about ABS you can cheat the system by wiring your ABS light on the dashboard to the battery light and making sure it is inserted so not touching the contacts. That way it is off from the perspective of the ABS system but "on" when the lights are checked by the MOT tester.
Dodgey George, March 2008
