Sat and passed the assessment back in February. Have a look at Skytest, this will prepare you well for the aptitude test. The interview is a general HR interview (give me a time when you did/experienced/led etc) with the usual follow up questions (how did you feel/overcome/view etc). Be yourself in the group exercise. Overall, I enjoyed the day. I took the offer of accommodation that they provide on site for a night and was very happy about doing that - was well rested for the day and the breakfast they provide is excellent - Full English and a continental buffet.