Black tie means either cocktail or floor length are equally acceptable. Do you have something in the wardrobe which would work but that perhaps you need accessories for? Or, if you want something new, what's the budget?
My top tip is to not buy from Coast as it is one of the most popular high street choices. Having said that, if you choose something plainish from there then you can put your own individual spin on it with accessories but obviously be prepared for the doppelgänger moment!