black tie at 40? help!

[hatwoman]

I've just realised that an event I'm going to is black tie. I'm loathed to buy anything new. My existing choices are

• long (ie floor length) black dress - it's made of two layers of a light fabric which very slightly frill at the hem. it has a low-ish scoop neck and very short sleeves with a slight frill. it's gently flaring, very flattering, and, although several years old I think it's reasonably classic.

• a coast outfit - their standard wedding stock which I think could lend itself to black tie - bustier with a fabric belt and straight skirt (when on it looks more like a dress than separates) It's dark grey with a vague brocadey type pattern on it in teal. (which is nicer than it sounds - the two colours are actually very close and tone together nicely)

the other people at this event will be similar in age to me and I strongly suspect few will wear anything long. but really not sure. any opinions/words of wisdom (or, even, links to cheapish possible alternatives). I thank you.

just remembered I also have a floor length bias cut dress. top bit is dark silvery satin, then it has a diagonal seam (kind of from hip height to below the knee) and underneath that is dark grey non-satiny fabric. high round neck, sleeveless, possibly a bit too clingy round the bum.

anyone?

fwiw, i would go for the Coast option. most black tie these days is cocktail dresses i think (I dont have huge personal experience, my sister does tho!). That sounds dressier and nicer for that kind of thing.

but beware, check dress code with organiser, i once failed to and turned up overdressed (in cocktail dress for formal evening dinner when all other women were in business suits) and felt very uncomfy all night.