Can anyone advise where I should start looking to solve this? I was told it was one bumper sensor and replaced it to no avail (maybe a fault cleardown is needed?). There is a main control unit for the sensors somewhere. I get this fault immediately after I select reverse gear.
You say "one bumper sensor". I guess your car has multiple bumper sensors. There is a way of checking them one by one without any special tool. If in silent environment and with your ear close enough, you can hear a properly functioning ultrasound sensor making silent yet audible ticks.
See this video.