When I stick my car into reverse, the reverse sensors come on but just do one long beep of about 0.5 seconds, then two very short beeps, and it carries on this way, it no longer senses things.

Anyone know where I can get the manual for it so I can find out the problem? I believe it may be one of the sensors, which sounds expensive if I had to get it from ford.

The sensors are the ford dealer fit XVision XP340R if that helps.

If it'll cost loads to fix then I might just disconnect the system temporarily and buy one of the cheap ones online.

try unplugging the sensors one at a time, this will tell you if its a dodgy sensor,there are plenty of places that sell x vision systems even ebay,or as you say you could put a complete new system in, the kits are not expensive nowadays, this company say that they sell the replacement parts if needed.
http://www.caraccessoriesplus.co.uk/product/ford-parking-sensors-xvision.html