Hope you havent had it yet, maybe it will help others in the same position. The interview is not too bad. This said, i had already been offered a place in the NATP when i walked in... so i was pretty relaxed. You do a 'potentia' test, which is like an inverted case study, as you get less than 3 paragraphs of info and you need to talk about it for about 35 min. not easy, it all depends on your knowledge of the world and business acumen.
the sencond part is an interview with a manager \ partner. this is very similar to the first interview, incorporates some case study etc. dont sweat. there is nothing that you can do to prepare. the only advice i can give is be confident, outgoing, and think MANAGEMENT. think big. no small. if you know econ, think macro supply side and demand side decisions, prices, governmnet spending etc. good luck. out of curiosity, if you passed, when do you start and where?