In addition to the information on this website, this article from the Good Schools Guide gives some interesting insights into the interview process. It is well worth a read.
There is absolutely nothing wrong with doing some extra work over the holidays to consolidate or reinforce a particular topic, or simply in order to keep things ticking over. In fact, when exams are
Never underestimate the importance of the interview as part of the pre test process
Practising questions is important, but simply becoming familiar with the format, layout and "feel" of past papers is also crucial.