Grown-up posh lunch outfit needed!!!

[RabbitsarenotHares]

Hi everyone,

This is probably going to sound a tad pathetic, but I need some ideas. I'm going to a silver wedding lunch in a posh hotel in a couple of weeks and need something to wear. No problem, except I have to see family members there who treat me like a child (I'm in my 30s, but that makes no difference to them) and make me feel worthless and insignificant. I was discussing this with a friend who suggested that if I dress the part it might help me believe in myself, which might help matters. And I'm stuck. I really have no idea.

It's a lunch in a posh-ish hotel. I probably want to go for a sophisticated look, but on a budget. I'm willing to spend money, but would see if there's anything I already have that matches any ideas you might have IYSWIM. I'm hopeless with makeup, and although I might be able to get a makeover in store beforehand I can see myself not having the time, so it'll probably be very basic foundation and eyeshadow / lippy. I'm fair, with short, dark blond hair. Size 8/10 (but with a tummy), 5'4.

I know this is a random request, but any ideas?