Hi captyous
I recently went for stage 2 (about 4 weeks ago and have subsequently done 3 and 4) and what you can expect is what I imagined it to be.
A sums test (get practicing doing your sums without a calculator!) which obviously is designed to put you under a little bit of time pressure but not too much!
A group discussion around one of three current affairs topics which was interesting and actually enjoyable.
And the big on is the computer aptitude tests. These are OK but hard (obviously, they don't make any excuses for having tight selection criteria) but don't worry too much if you feel like you've done really badly when you finish. The whole room thought they had failed when I did it and I went to the next phase with three of them!
Good luck and make sure you get a good nights sleep and a decent breakfast!