I went to a black tie dinner where men were invited to wear regimental dress and every woman wore long except (get this) the guest of honour, some Colonel or other in regimental dress whose wife wore black trousers and a grey cardigan! No-one batted an eyelash but I can remember feeling insulted that I had made a lot of effort and the wife of the guest of honour made none at all. Other dinners I attended were a mix of knee length cocktail dresses and long. However you dress, make sure you act as if you did it exactly right. Cardi-pants did and got away with it!