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.
I could not hear anything, but thank you for the suggestion.
the answer is that they are simply wired and do not have an id. My solution was to buy a cheap laptop and Diagbox and solve it myself. If you run the trouble shooting it identifies which sensor is faulty although the translated french which indicates the positions is a little unclear and refers to left hand drive. I found that its easiest to rotate the sensor until the lugs disengage and then gently push it through from the outside as internal "grab space" is limited if you have big hands like me. I bought cheap lookalike sensors from ebay and they work perfectly well.