Tests.
I found the following book very useful:
'Psychometric Tests for Graduates' by Andrea Shavick
All the test examples within are by SHL Group, who are the company responsible for setting the NATS tests. A happy by product of this is the examples are identical in format to the ones given at the assessment day.
I'd be more than happy to let someone have the book free of charge as i've already done the test day. Anyone who wants it PM me with where you want it sent.