Mate, don't get involved with any of it, I can't be arsed to read all the replies above so no idea if you already said this:

You know your rights, he has to give you the opportunity to fix it yourself.

Tell him to contact the Office of Fair Trading if he has a problem and I'll bet the £100 you will never hear from him again.

I wouldn't have even offered him the Lamda sensor, who's to say the garage didn't fuck it up when they were doing the other things?

Contact the OFT yourself and get a reference number if you have to.