Park Distance Control up the spout!

[grf500]

Stuck the car (330 Coupe) in reverse and heard a long beep as though something was behind me (even though nothing there). Then I carried on reversing into the space but the beep stopped and was completely silent - no beeps. I checked the sensors and they looked ok so I wiped them clean..... then they didn't work at all!!!

Anyone else had a similar problem? Can I check something out myself or does it have to go back to the unhelpful/expensive dealer?