This has something that has been on my mind and someone mentioned it in a thread before but how do these actually work?

Does someone independent actually come in to do them or does Arriva just do MOT's for well Arriva etc. Seems a serious conflict of interest effectively doing your own MOT's and is kind of a bit pointless if you had questionable practices. Not saying anyone up here would do that, was just an example other than Rural Link or whatever name they are this week who I wouldn't put it past.