I had a PhD interview in Cambridge. The panel focused on asking very detailed Qs about how I intend to spend the three-four years of my PhD. Details like asking my experimental design, Qs I want to answer, why the Q need to be answered. That sort of thing. It was hard but I think they want to see that you have a good knowledge of the field and a real interest. My interview went well. Smile, be confident and don't feel you must know everything. Good luck!