OAP Pete
As an EX Auto Electrician [ now retired ] these faults can be caused by just the Bulbs....yes the lights may be working... but after some years the bulb resistance does change... re-the stop lights the ECU control for the Display on Dash will be looking for 42watts, this wattage does change over a time, do change BOTH bulbs first , check the multi-plug on the rear bulb holder, it could well be corroded, must be clean. These lights as i remember let in water and corroded plug, This above applies to Head lamps etc. Hope of some help. Pete.