Regardless of what anyone else says this "mechanic/mate" should have given you a realistic estimate. I would say that £15 is a bit cheap just for the belt. But you don't really have any way to go other than pay him. You could take it to court but that would, probably, cost you more in the long run. You could not pay and let him take you to court. Personally I would pay up, grudgingly, and then tell him to go stuff himself and never darken your life again.