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!!

[Big E 118]

They would turn on when placed in reverse.

One faulty sensor will knock out the whole system, do you get a constant tone for a couple of seconds when the car is put in reverse?

[SPR117T]

I don't get any tone at all