Go in beforehand, see what the staff wear and copy that? It's what I did when I worked in a club. Failing that, can't go wrong with black jeans and a dark t-shirt/vest top depending on the atmosphere (and weather?)
From experience in Walkabout (Used to be a semi regular in Doncaster), just a nice pair of jeans and a casual black top should do. They probably won't judge you badly for picking the wrong clothes, they'll be more interested in how you handle the job.