I have (Audi fitted) after market rear parking sensors fitted to my 57 plate A6 Avant but the little monkeys at Longleat found a couple of them irresistible. Can I get just the replacement end bits or do I need to look at a whole kit??

Depends on the kit.

 

Most aftermarket kits do have separate sensors that plug in so you should be able to replace them.

 

If you feel behind the bumper on an existing one you should hopefully be able to follow the wire back a short distance to a plug. Then it's just a case of finding our which brand of sensors you have and ordering the sensors.

Most of aftermarket sensors can be installed separately :) Then replacement is easy :)