I know you've had plenty of advice by now, but here's some more from a future trainee!
"My top tips would be to know the firm well (products/services they offer and how this sets them apart from their competitors), think about their international expansion (in my interview I was asked a lot of questions about this), in the interview (which was a member of grad recruitment and 2 partners) just be genuine and show them why you will fit the firm. The questions were scenario based and competency based mainly so it is key to find a way to incorporate knowledge of the firm into the answers.
From what I have heard they have changed the AC since I did mine but in the group activity I had I would say the tips would be to show leadership skills when needed, communicate well with the others, make only valid points (do not just say things because you think you need to speak as much as you can!). I also had to do a presentation (there was a list of topics to choose from which you got on the day). Tips would be use the whiteboard/paper you are given, make sure you stick to time limits, if you do not have much knowledge of the topic then that isn’t the end of world as they are focusing on your presentation skills (eye contact, speed of speech etc). The final activity I had was a written test, I don’t have many tips for that that I’m afraid, the instructions they gave were easy to follow so just make you you plan your time well to get it completed!"