If it's any help to you, I went along in dark blue Jeans and a jumper. There was only one other person who I would describe as casual.
I have passed the tests and been invited back for a interview or two!
I will definetly be suited up for that one though.
Your sitting tests, not a personal overview, the advice on the weblink (posted in the top of the forumn) which is dress how you feel comfortable seems appropriate for Stage 1.