The ABS modulator is shared with the Disco 2. Basically there are flutter valves in it which, if you don't use HDC or ABS, jam shut after a period (it was common in 4 year old Discos that had been used solely on road) so during the test at start up the computer recognises an error, hence the lights.
I don't know if this is a cure, but you may as well try, but I do know it works as a preventative... activate the HDC. The easiset way to do this is to press the HDC button, accelerate in 1st and then let your foot off the gas pedal. The computer gets 'scared' and applies the brakes (with much grinding noise) but all the flutter valves are operated so they don't become stuck.
Don't know if it will work once the warning light is on (HDC may have been disabled) but good luck... they're expensive units to replace.
I've heard of technicians opening up the units and cleaning them but I don't know how much it costs or how effective it is.