Re: 156 Service
Take the car back and enquire why a new noise is there and why there seems to be less power. Ask to speak to the fitter who did the work. Also take a quick look for any little yellow splodges of paint or crayon. These indicate a YTS student doing the work! I joke not!
Another approach is to discuss the problem with the garage and then get the manager to write a signed (on headed paper) note saying that if anything goes wrong which is cam belt fitment or failure related, they will agree to pay the full cost of a new engine being fitted. If they don't agree to this, ask why not? Not threatening, just enquiring!