I would also recommend checking with the organisers about the dress for the Burns Supper in question; most Burns Suppers are a mix of dress, with full Highland formal to "Saxon" black tie and "church" dress (ie a nice suit). If it is a more casual, "pot-luck" type Burns Supper, you could easily get away with a nice sweater, or a sweater-vest, shirt and tie.

Regards,

Todd