There will be a good introduction to the scheme itself.
Be prepared to make a presentation about yourself to other candidates and CTC staff (you dont have to stand up!)
Candidates will be broken down into two smaller teams to complete topical group discussions. A number of topics will be introduced by staff.
Preparation? Get upto speed on your mental maths (forms part of the computer based testing), hand/foot/eye co-ordination, etc.
There was nothing too technical when I was there (i.e. no tests on AGK). Might be asked to analyse various airport approach plates.
Thoroughly enjoyable day!