Just to add to this, I think I know which the bad sensor is now. It seems the comment listed about the sensor pulsating works on the factory fit options too. Put the car in reverse and you ca feel it "ticking" away, and can even hear it if you listen closely. There is one next to the number plate that doesnt do this and wondered if:
a) anyone know the part number (easier to order the right one and change it over when opened up rather than doing it twice)
b) does it require the top part of the bumper to come off too or just the bottom part and reach in to swap over?
TIA