seriously, take Oxbridge Tutor's advice.
i went to my interview wearing jeans, a really well tailored blue shirt, and New Rock boots. I sat the exams wearing jeans, a dress i'd bought 17 years ago and the same new rock boots. they didn't once look at what i was wearing, and i felt i could have shown up in my pijamma top and they still wouldn't have cared. they want to see your brain, not your fashion sense.
show up wearing something which makes *you* feel confident. if that's a suit, so be it; if that's jeans, do that.