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.
http://www.parkingsensors.co.uk/dolphin_display_reversing_sensors.asp
They also do a mobile fitting option if you prefer - which i did. Didn't fancy letting myself loose on her brand new car.
IMO they are worth the money esp if you have to parallel park in tight spaces on a steep hill like we have to.