aphelio:
Stage One booklet can be found at
http://natscareers.co.uk/docs/test_materials.doc
Advice for stage two is don't panic during the computer based tests! Everyone goes away from them feeling like they've messed them up. Go through the stage one booklet, if you weren't sent it, and also look at
this which I was asked to learn for my stage two. HR suggest you spend between 8 - 10 hours learning the booklet.
That is the only preparation you can really do. The structured interview, as has already been mentioned, is a set of questions HR asks everyone. They will not explain what the question is trying to get you to say, nor will they respond to your answers. All I will say is just think of the type of questions they will ask you, do you have examples in your life where you've demonstrated leadership, teamwork skills etc.