Hi!
I'm at Surrey doing engineering now but I expect your interview will be similar to mine, I had the presentation/interview day thing. I think mainly it is about getting a feel for you and whether you would be right for the course (ie. do you have a clue what it's about and are you interested in it). They ask you about your personal statement and just have a bit of a chat just to make sure that you know what your course is actually about!
I was told on the day that "the fact that you have been invited to have an interview means we will probably give you an offer"...I can't speak for Maths but you can make of that what you will.
Good luck!