Audi A8 D2 Parking Sensor woes

[Audi Doody]

Hi all,

So my parking sensors started to play up, this could manifest itself by them not working at all, or a long beep being heard.
Got it plugged in for diagnostics, and this identified that it was the beeper inside the car that was broken; all of the actual sensors were ok.
Is this a common fault? Apparently the sensor is located behiind the instrument binnacle, is the sensor on its own, or is it part of something else - in other words, will it be expensive?
Many thanks