A colleague of mine went last year so hopefully things haven't changed too much...
The interview is in 2 sections - the technical test is an ATPL style exam covering all ATP subjects. 40 or 50 questions in 30 minutes - fairly easy if you have a 'quick' look through your ATPL notes!
The verbal part will be with 2 interviewers, lasting about an hour or so. The majority of this is talking about your career to date, where you see yourself in 10 years and finding out what you know about the company. There are also a few technical questions to make sure you know your facts about POF.
It's supposedly a fairly relaxed affair with no real tricks or surprises.
BTW, what experience do you have and when did you apply?
Good luck to everyone going!