Rover 620 sli auto abs and brake warning light stay on?

I have a rover 620 sli auto 1994 L reg
when starting the car the brake warning light (!) stays on and once driven a short distance the abs light comes on and stays on .The car has had front and rear uprated discs rear discs are a modification to the usual rear drums but work was done before i purchased the car. All fluid levels are correct and braking seems to function correctly any ideas please