Hi ajl,
I went down for my phase 2 about 18 months ago now so I'm not sure how much use my information will be to you. I'm pretty sure CTC have now combined the group exercises from Phase 3 with the initial phase 2 aptitude testing.
I can only comment on the aptitude testing as I decided not to pursue the CTC route any further due to other commitments. Prepare thoroughly for the maths tests. Cover long division, long multiplication, unti conversions, and really try to do as many actual problems as you can, rather than just simple questions (e.g. 76880 divided by 567.5). Time management is key, it will be 15 questions in 15 minutes, which is more challenging than it sounds in my opinion.
As I say, it was a while ago since I attended, so things may have changed. There's heaps of information on this forum specifically about phase 2, so get searching. You'll find what you're looking for.
Good luck,
CF