I had this on a '93 420. It was a failed wheel sensor, which I gather is one of the most common faults with ABS. Connect a multimeter to the plug from the sensor (down by the wheel) and spin the wheel. It should give out a voltage (about 1V AC, from memory). Check which wheel isn't putting anything out.
The sensor works, I believe, by inducing a current as a toothed wheel on the hub rotates past.
I seem to recall reading about it initially on
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and have seen something on the
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discussion groups as well.
The sensors are ferociously expensive from Rover (about £170 with the mounting bracket), so I put a request onto one of the find-a-part breakers rings in the yellow pages and found a 2nd hand one. Mk1 400's and Mk2 200's with ABS are pretty rare so I had trouble in the normal breakers. I ended up with the entire hub assembly from a 200 Coupe and swapped it whole (having swapped the spring/damper back to my old one, of course) as it's almost impossible to remove the sensors once they've corroded in. It cost me about £35 as I recall.
Good luck! Adrian
Not been crashed either I might add.