Agre with Flum that a full-length dress is not called for. I would think more along the lines of the type of dress that you would wear to a wedding would be suitable - something knee-length, maybe sleeveless with some frills or flounces to dress it up and a pashmina to keep you warm. Like the kind of thing that Monsoon do well - does that make any sense?
I'd day no to the trouser suit unless it is something really special. Knee-length dress (not full length) would be what i'd go for, something beautiful but not too showy - could be dressed up or down dpending on ocasion - you may get some more info about what others plan to wear closer to the time and pick up a bargani int he July/August sales.