SKOS,
I believe the invite to interview email should tell you, but it'll only be along the lines of "smart casual", which can be a very broadly interpretted term.
TBH, I wore a suit to stage 1 (even though I wouldn't classify this as smart casual) and was glad I did - everyone there on that particular day was dressed similarly and I certainly didn't feel over dressed. Off course there is no interview at stages 1 or 2 so arguably it doesn't really matter, but personally I'd rather play it safe than look a scruff (and you never know who's watching...). You could always dress smart and if you get there and realise you are the only one then just stuff your tie in your pocket and undue a button or two of your shirt.... and like magic you are now smart/casual.