I'm waiting for my results from the level three interview. I found it's pretty free ranging and I got the feeling the interviewers like to focus on any grey areas in your employment and education to find out what actually motivates you.
So as well as prepping with the NATS materials I'd advise practicing some run of the mill HR type questions because I think only about 20% of my interview was based on the NATS reading materials.
Although it seems the interview may be different for everyone based upon their background and experience, the very least preperation you should do is to learn the provided reading inside out.
I'd also strongly recommend some further reading (eg Graham Duke) which will help fill out your knowledge a bit, and visit at least one towers or a control centre.
If you're challenged with a question and you don't know the answer, avoid the b*llsh*t and own up. Favour safety conscious answers, and stand your ground, unless it has been pointed out that you've been barking up the wrong tree, in which case take it on board.