Gavin,
If you have a read on this thread there is a lot of info on the selection day but a brief introduction is as follows:
Note: I completed the selection process over a year ago and I believe its changed since with selection now completed at Dibden as far as I am aware. Also I believe stage 4 has been dropped from info I have read on this thread.
Basically you turn up and get a presentation about CTC before being split into two groups. One group completes the pilapt tests and maths test while the other starts the team building exercise etc. When each group has completed the tests etc you swap over. This will take up to lunchtime and the results are processed during lunch. The group that has passed will have candidates called forward for stage 3 interview and the guys that don’t get through will have a quick exit interview. If you past the interview in the afternoon you used to get called for stage 4, a sim ride at Nursling but I believe this has now stopped.
Hope this helps.