This is another matter for the data protection act. ANY company holding information on you is obligated by LAW to allow you to see that data apon your request. With some expections like the police or government.
Remember all those notes the interviewer was taking, and what about the over all score you achieved, and your weak points. That is all data held about you and you are intitled to see it. They may charge you £10 to send it out to you but at least you will then know where you went wrong.
So next time you are on the phone ask them to send you a copy of the interviewers report.
If they say no sorry you are not allowed to see that. Then they are breaking the law. However accusing them of that and pushing them may have an effect on your chances of being successful at another time. Its a gamble I suppose but all you can do is ask..