Not exactly the same, but I was lucky enough to get onto a cancer research PhD programme at Bristol uni last year.
Most of the discussion focused on my previous undergrad research experience; why I used the methods I did, what experiments would I have done if I had more time, what did my time in a lab teach me about the scientific process in general...things like that. Some of the questions were quite technical, so I advise making sure you know any research project you've previously undertaken inside-out.
Personally I wasn't asked anything too bizarre, but I did hear stories of other people being asked questions like, "if your best friend was here now instead of you, what would they say about you?"
As for what I wore, I went for smart casual - smart trousers, shoes, a reasonably formal shirt and a smart jumper. Not many people went full-blown suit to be honest, I think it's an unspoken rule that people working in academia are generally a bit scruffy.