Wedding evening do - what to wear?

[notsureconfused]

What should I wear to a wedding evening do? I can't decide if it should be something less formal / fancy than I'd choose if I was going for the entire day? Is a party type dress too informal? Please help!

Work on the thinking that you are joining guests who have been there all day and will be in their best bib and tucker.
So wear something you would wear to a wedding, but without the hat / fascinator.
So, if by a 'party dress' you mean a 'clubbing dress' then yes, you are right, that isn't the right 'look'.

this is for most, fairly traditional weddings - of course, some folk have all sorts of different weddings, from black tie and evening dress to jeans and wellies, but I'm presuming as you've not said otherwise, this is fairly traditional.