When I did it back in 2003 most people turned up in smartish civis (i.e. shirts with collars and jeans/trousers although a few did choose to dress down slightly)
As for other occasions, simalar standards were expected on the battlefield tour (mine was in week 9) and your interview with the Personel Selection Officer for the Corps you were joining was all done in suits but some troops didn't have that for whatever reason.
But yeah as said before turn up reasonably smartly, it is a new job after all and you don't want to be upsetting the training staff before you start and keep a (small) supply of smart clothes for whatever might come up. For example on the one occasion my troop was allowed down into the local town it was only those who had shoes, rather than trainers so a lot of people weren't allowed out.
By the way if you've got a kit list telling you to bring loads of shi@ like own pt kit, more civiy clothes etc don't bother. your supplied with a "tracksuit" when you get there that and your combats are the only thing youll be wearing whilst there apart from when you do your battle feild tour.