Update: I took the car to Kwikfit for a free exhaust check. Within 5 minutes they found that a weld had cracked just in front of the back box. I took a video. I found a specialist exhaust centre who fixed it for £60. I arranged a meeting with the head of the service department at the dealership and gave him the whole story. He saw the video. He was very apologetic and agreed that I had not had the quality of service that I should have, in fact he referred to it as a fiasco. I was offered £200 off my next service, which I felt was generous as I had not asked for any compensation.