I did this interview last year but was unfortunately unsuccessful, as I remember it your day will go something like this:
Arrive at 8.30ish for a pre day briefing on what is to happen. The morning will be taken up with 4 tests? consisting of 2 maths tests, 1 English test and a mechanics test. You cannot use a calculator for the maths tests and some of the equations revolve around working out Sin, Cos & Tan. I borrowed an O level maths book which helped me out here.
Once these are done you will then be required to sit several tests on your jack, these include the standard computer aptitude tests, a logistics test (ie an oil rig has 2 hours to live, you have to evacuate 20 people with one helicopter 20 mins away the other 10 mins away but requiring maintenance etc etc). You have 10 mins to work this one out before presenting it to 2 of the instructors. Hint follow it through to the end, don't just get the last people off and say that the chopper can go to any of the locations, specify which one.
The final joy is the interview in front of Capt Bedford, the boss of the flying school and the head of groundschool. Start reading the Times or whatever, bone up on the technical side and don't let them pressure you. Good luck