BMW 120i SE Park Distance Control (PDC)

[SPR117T]

I have just bought a BMW 120i SE for the Mrs and notice there are reversing sensors on the rear bumper. I have an M3 with the same and in my car is a button marked P where I can manually turn it on/off. However in this car there is no button and the park distance control does not sound. It could be that it is automatic when reverse is engaged, however it still does not work. Seeing as the Mrs is a terrible driver, is there an easy fix. I know you can get a dodgy sensor, but nothing is working, it can't be all of them. Is there a seperate fuse, any suggestions.
HELP BEFORE SHE PRANGS IT!!

[Vic_S]

On my E39, just 1 x faulty sensor will cause the whole system to fail.

With the ingition On, select reverse and then listen for a faint "clicking" at each sensor. If you find one that is silent - it has failed/needs replacement.

[toys]

My 130 has a 'P' button to manually switch the PDC, so something sounds odd. Could it be an (incorrect) replacement bumper?