I went in a suit to stage 2, most other people there were dressed similarly so it won't do you any harm to over dress (you won't be the only one). That said, the one guy who did turn up dressed more casually (jeans and a shirt) was also at my stage 3, so it obviously didn't hold him back. Just don't turn up in a tattered pair of trainers and your favourite Metallica T.
It's quite an intense day at stage 2 as well, so make sure that you are comfortable whatever you choose to wear.