Hey Tom, I'm currently going through the Air traffic control selection at the moment (passed my first batch of tests at the facility just outside Southampton a few weeks ago).
All I can advise for that is make sure you can do Time/Speed/Distance type calculations fairly quickly in your head. Other than that it just seems to be a case of "you can do it or you can't" for the aptitude tests.
Good luck if you decide to go through with it!