Budget not really an issue. Only as in I don't want to spend £££'s as I probably won't wear again, and am also on a diet (have lost 22lb's) but have a way to go. Am now 16/18 but would like to be 10/12.
As for body shape I am an 'apple' with short legs.
Not sure about that top for black tie. I think a trouser suit could look lovely, but it might be hard to pull off in this context.
I wonder could you go for something like this? Only size 14 left in the black (I don't like the red, personally) but depending on the sizing it could fit, or they could order the 16 in for you. It looks like it would be easy to wear?
How about a session with a style consultant at John Lewis? It's free of charge, no pressure to buy and she will help you find something for this specific event that you like. John Lewis has lots of different labels so you can try all sorts of things.
I thoroughly recommend this service.
I love the idea of a trouser suit too and I love my brogues (from Clarks).
What shapes suit you, what bits do you want to show off and hide? What colours do suit you? Budget? House of Fraser have lots of evening dresses, I bought my fav from there in a sale a while ago. Or Monsoon? Phase Eight. You just need to go and try I'm afraid. Sorry probably not what you wanted to hear.
Could you be cheeky and do a fab trouser suit, as seen in all the magazines, you could wear with brogues maybe. Or get a simple wrap or mock wrap dress and wear with knee length flat boots, a clutch and a fab necklace?