What to wear on top of this?

A cardigan that you could wear buttoned up (it can get quite chilly in the evenings) or a jumper. Nothing too big and clumsily if you want to the look smart. Or maybe a smart jacket, if you're going to be outside. Try to match whatever you wear on top to the colour of the dress so you look co-ordinated. Because the dress is so cut away at the back and is sleeveless, I think you need something quite substantial (or maybe I'm just a wimp about being cold!) It's kinda hard to advise you though cos I don't know what type of thing would suit you