I would recommend taking a nice folder with all your certificates, ID documentation and everything else they require you to bring, it makes it look like you have made the effort and that you are keen.
I had a few clumsy moments, I didn't have my original Birth Certificate, only a certified photo-copy.
In regards to the suit, I didn't wear one, instead I work smart shirt and tie (good creased shirt) makes you look well disciplined, firmly pressed trousers and polished shoes.
Like many have mentioned above, they are a big recruitment agency, make your first impression count. Think ahead, try not to ask silly questions and of course turn up on time, I had a few candidates at my AST turn up like they had just woken up, they were later dismissed.
Overall, the very best of luck.