Hi Liam,
Opinion varies, but I'd suggest that there is certainly a degree of preparation you can do for stage 1:
Read and learn thoroughly the document NATS send you - there is an entire paper on this, so if you don't know it then you will fail! Also well worth brushing up on your mental arithmatic if you don't use this a lot in your current line of work (check out
www.speeddistancetime.info for some topical practice examples).
Some people find it beneficial to practice the cubes and the diagramming tests, but example questions can be quite hard to find and many people end up devising their own. Personally I didn't bother with this, but I've always found that these type of tests come naturally to me and I know that for some people spending some time practising can be very helpful.
Good luck