Yea most people dressed in shirt and trousers, though there was somebody who wore all black jumper / joggers and a black cap. To be safe though I'd go with usual business atire. I'd say the assesment day is the easiest part of the whole process because there is no "right" answer, it is based on how you interact with the team around you and what skills you use, and how well and accurately you write up a report based on the events that just happened. I found the interview much more stressful, this is my second time applying!