try cleaning the sensors
Had this, easy one to try. Switch ignition on, put it into reverse. Now stick your ear down to each sensor and check that you can here a clicking noise. If one is not clicking then it's faulty or the wiring has come lose. Check wiring, if still don't get any joy probably have to replace the senosr and they are not cheap.
Can you get at the rear side of the bumper from inside the boot at all?
When you initially put it into reverse is there supposed to be a beep think it's 3 times, its a different sound to the reversing sensor sound?
Daft question, are you parked near an object. My sensors go off when there's nothing there and when i'm near a car, wall, person, they go mental.