Pez, I think you must have missed the interview tehnique messages here for there are many.
They will want to know about you, school, hobbies, etc. What you have done to further your interests in a career in the Royal Air Force. A high point in your life - and why. They will ask about the RAF to test the depth and breadth of your knowledge. Don't worry, they will expect you to know less than a serving airman although more than someone who has not been in the ATC.
They will be looking to see how you have used your opportunities - what rank and responsibility did you achieve in the ATC. Were you a team player or a team leader in sports etc.
It is all here in the thread.