Simon,
My best advice is to trawl all the threads on the ATC issues section. There is a wealth of information on there inlcuding all the latest on what you can expect from assessment days/interviews. There's also plenty of current applicants going through the process that you can get chatting to and they will be able to guide you through it all.
To regurgitate it all on here would just be repeating what has already been written elsewhere.
Best of luck with it all though!
Regards,
DS