Hi Chris,
From what I've read in this thread, Stage 1 (Part 1) is mainly about testing your motivation and your math skills. What everyone suggests regarding the former is to learn very thoroughly the 25 pages NATS sent you because it will be useful in the first paper. Also, if you want to go the extra mile, I recommend reading Air Traffic Control by Graham Duke. It's like 110 pages so you'll read that very quickly and it is very interesting. It presents basic stuff of the UK airspace and also many things concerning airport procedures and control centre operations. It is also easy to read, at least if you're interested in these matters
Regarding the math skills, have you tried the Speed Distance Time Tests for RAF OASC candidates website? They also have a fuel calculation test and an orientation test.
Good luck!
