beaky-my best advice is to get in touch with the ODCA and see what their advice on the issue is-I can't say for sure myself what your rights are when repairs are undertaken without your authorisation (it seems to be fairly common in motoring circles).
I guess if you don't dispute the fact that the part needed to be replaced, you should pay for both parts and labour.
If you don't want the part, tell them so, get them to take it back and leave the car in the same position as if the new sensor had never been fitted.
I don't know if you can take the 'middle ground' so to speak, i.e. take the part but not pay for the labour.