What would you wear to a black tie dinner as a 47 year old not-slim woman?

[OlgaFjeldso]

DH and I are invited to a posh evening do, drinks then black tie sit-down dinner.

I will have to buy something new to wear. It’s in May so weather could be hot or cold.

I don’t like floor length dresses, would prefer knee length or just below (but perhaps not formal enough??)
I don’t like high necks.
Prefer something a bit structured or fitted, but I am quite curvy.
Have shoulders and upper arms like a scrum half, so might be better covering those up to attempt elegance - maybe a dress with sleeves or some sort of jacket/shrug/bolero over a sleeveless dress.

Don’t want to spend loads for the sake of it, but on the other hand I hardly ever buy new clothes, and I’d like to look nice.

Any help would be appreciated.

@Darkclothes
I cheat by wearing a strapless swimsuit under form fitting things. Holds all together.

In my experience there are several different types of black tie event. I the last year I have attended three types:

The business function (think prefessional society annual dinners) where there is no dancing involved and no couples - strictly business with clients and colleagues. For these I would opt for something more formal between knee and floor and not exposing too much bare shoulder or other flesh, closer to a cocktail dress, but longer than the traditional cocktail dress and maybe a little more extravagent.

Summer outdoor events (e.g. opera), again without dancing, where I would opt for a ballerina length rather than full floor length as that could be tricky when navigating steps and the gardens etc.

Balls and charity black tie events with dancing - for these I would opt for floor length or only slightly above. It is fine to go sparkly and all out as long as it is tasteful!

In my view, knee length is not long enough and the fabrics need to be more luxurious or have more embellishment than cocktail dresses (without being gaudy!).

University balls are slightly different as the young graduates get away with wearing just about anything!