I'm going to sound like a freakoid, but here we go, confessions time ... when I went for my interview, I made a conscious effort to look as much"the part" as possible, to make it easy for them to picture me in the role ... that meant a darkish blue/grey suit, perfectly ironed mid-blue shirt, 15-denier barely black tights, smart but sensible well-polished black heels, and hair in a very neat bun. I like to think of it as paying attention to detail!
OTOH, the first time I wandered in off the street, I was only in a tweed skirt and jersey with a winter coat (God, I sound like a complete sloane!), and the cpl said there were no issues with my standards of dress ... so certainly don't panic if you've already been in without a suit!