Thanks p_square. After posting and before seeing your reply, I had a bit of an experiment with putting a camera tripod (fairly chunky head) behind the car about where I imagined the towbar would be-and yep no detections. But if I moved the tripod a bit further away, the detectors would go off. Which is a good thing if you are about to back into a low bollard. See how it goes for real in a week or so.