Kevster,
it's nearly seven years since my interview so someone else may have more up-to-date info.
anyway, basically a "technical" interview conducted by two ATCOs who will ask questions based on the info you've already received and talk through some scenarios with you. These scenarios will probe your knowledge more deeply, they are not expecting you to have all the correct terms, phraseology etc, but to give a logical reponse to what has happened.
also a personnel type interview which can be quite probing about your previous history, in my case I was given quite a grilling about my less than illustrious education record.
they are likely to ask about your feelings about shiftwork and to relocating - remember it is a mobile grade so if you've got a major dislike for a particular part of the UK then keep it to yourself.
finally I had a series of computer based tests which involved a number of arithmetical problems flashing up on a PC screen and for you to give the correct answer via the keyboard before the next problem and also a vectoring type exercise where you had to keep a number of blocks on the screen from hitting each other.
hope this is of help