OK, just to clarify for those going through this process now - from my experience....
Stage 1 - you are a number, you can wear whatever you like and no-one will ever know. I (and most) were wearing jeans and t-shirt for stage 1.
Stages 2 and 3 - you are being interviewed so dress accordingly - ie not jeans and t-shirt!
Also - there is no negative marking in the tests - if you get the answer right, you get a mark, if you get the answer wrong, you don't, simple. So just a suggestion, make sure there are no blank answers on your sheet when the time runs out, as they'll definitely be marked wrong - a non-blank answer has a 1 in 4 chance of being right!.....