Good luck mate, you won't have your first formal interview when you take your barb. It's just barb and if your lucky they'll get you to do your maths and english tests same day so that gets them out the way. As for dress, wear a suit. I did my test with four other guys and they all turned up in jeans and t-shirts, I wore a suit and got approving nods from my ca. Shows your making an effort.
Good luck anyways!