Just picked up one of these online for £49.98 delivered. They seem to be offering 10% off as well so it's a billy bargain.
They get a decent write up at AutoExpress and may suit you if you dont fancy messing around with ultrasonic sensors where you need to drill holes and get an exact colour match.
The blurb from the Halfords page:
The Meta SR2 Targa Sensor is an easy DIY fit parking sensor. It is designed to mount behind the rear vehicle number plate while minimal drilling may be required for some vehicles.