Marc before everyone rips into, I suggest you read through this thread it tells everything about the day.
There is also a description of the selection stages on the CTC wings website as well.
This is how stage two works:
20 minute presentation about the partner airlines and an overview of the scheme should you pass all four stages and get to go to NZ/Bournemouth.
After that you then go into the assessment room and do a simple maths test, 15 questions in 15 minutes, this part you can prepare for. Make sure you are sharp on long division and multiplication, know your square numbers and know how to rearrange basic equations.
After the maths test you go straight into the PILAPT testing.
This consists of 7 or 8 tests that last about an hour all done using the joystick and keyboard in front of you.
If you go on the PILAPT website there are some screen shots of the tests. These are aptitude tests though so you can't really prepare for them, you either have those particular abilities or not (so they say).
Good luck marc