I bought a Cisbo system off ebay (after a bit of reading up on various forums) which cost the grand total of £20 delivered for colour coded sensors
I tested it in the house before fitting and it was incredibly accurate - warnings started at 1m and continued in steps until 40 cm with the constant beep kicking in at 20cm.
The spacers for the sensors don't fit (follow the contour of the bumper) 100% accurately (I'm talking less than a couple of millimetres out so they are close enough to look original) but I'll tweak them a little tomorrow - it may be that had I fitted them in a slightly different position they would have fit perfectly but I like them where they are.
If you want details of the ebay seller (who provided superb service) drop me a pm
Did a similar thing, but with a reversing camera hooked up to my headunit....
Select reverse and the display screen shows a rear facing image with an overlay of red / amber / green lines for distance....very accurate and it mounts just above the number plate and between the number plate lights...
no more running the cat over on the drive (wel, nearly anyway)