Hi John, yes a diagnostic should point you to a failed sensor, also check fuse 31 (15amp) in fusebox inside the car, this is for the reversing lights, if the reverse light system is'nt working, either the fuse or the switch on the gearbox have failed, the reverse parking sensor system will not activate.
If you would like a copy of the wiring diagrams contact me on nige110162(at symbol)hotmail.co.uk
nigel, November 2011