OK thanks to advice on here I have a nice dress for graduation. It's a work/formal dress from Next. White, tight and has a bit of a pepulum or whatever it's called, so basically quite fitted.
It is however from a outlet store and they only had a size larger or a size smaller than I normally take. I went for smaller (as Next is big sizing generally ) and it's tight enough that you can see my belly button dimple (goes in).
Do I...
a) Go for a size too big
b) Wear special underwear bearing in mind it already has a "control lining"
c) Hope the graduation gown covers it and be happy it was less than a 1/4of selling price.
d) Not eat for 3 days (have not eaten today today if it helps).