Dealer wrecked my car.

Hi all thanks for reading.
Well let me start, I currently drive a Seat Leon cupra 2008 model which I bought from a franchise Seat dealer around 11 months ago. The car has been excellent it's been 11 months of trouble free driving really.
Anyway just two weeks ago I took it to my local seat dealer to have a minor service done to the car but whilst I was driving back home I'd realised that the car was rattling when driving at low speeds the noise was definitely not there before. I rang the dealer straight away and explained to them that something wasn't right. The next day I toke the car back to them to have a look at the car and the service guy said that the noise was coming from the cam follower area and advised me to change it. I wasn't too sure that the part he advised was related to the noise but he assured me it was. Anyway I just had the new part fitted and my car still rattles.
As the fault is quite intermittent it's quite hard to replicate it all the time. I also ask them to check the car but he was too positive that it's was that part but now it isn't.
Anyway I have now paid for the service and now a part I didn't even need.
I feel robbed and sick due to the fact that they have ruined my car.

Any advice would be greatly appreciated.