For stage 1 and 2 don't worry about that Graham Dukes book just yet. All the questions will be related to the pdf handbook sent to you (obviously the 2nd stage questions will be slightly harder). Some questions are very specific. I found it's not like uni where you have to understand the concept and answer questions with that concept. Some of the questions require memorising of exact facts from the pdf. Best thing is memorise all the data.
I found the Graham Duke book very handy for the 3rd stage. You wont need to remember things like the exact services provided in different airspaces but it gives a good overall picture of what NATS provides. Which they may ask of you. They may also ask you what you expect to study once you get to college.
Good luck with the cubes. You'll never have to do them again after this.