Intermittant ABS warning light.

[p2roverman]

No doubt this has been raised before, but I am getting intermittant problems with the ABS warning light.

Always works normally on test at ignition switch on.
After a couple of miles it comes on and remains on for the remainder of the journey, OR it will sometimes go off again.
Other days it comes on immedieately, but goes off later.
Some days it does not come on at all during a journey.

fortunately it behaved correctly during the Mot last week.

Intermittant faults are the worst to find and fix, but any clues as to where to start with this one would be appreciated.