I've got a Meta SR2 kit that fits behind the number plate.
It was really easy to fit & no drilling was required. It’s a very neat bit of kit & is hardly noticeable.
The little noise box sits in your spare wheel well & you simply cut into the reversing light wire to give it power every time you stick it in reverse.
The sensitivity can be adjusted & has so far been really useful.
I recently bought some sensors for the Mrs' new car and they were already on my LCR when i bought it.
I did some digging before i bought and there was one thing that I kept being told. The ones that sit behind the number plate or as a plate surround only work for objects that are directly behind and not at either side. I could be wrong but this was the advice given.
The ones on the Leon are the "4 pot" version so there are 4 small sensors in the rear bumper - i have to be honest i would prefer an invisible solution but this one works well and was already there - they are also colour coded and not easily noticeable.
The ones i got for the Mrs were the same style and also came with a LCD display to show you how close you were on each side.