Trouser based outfit for black tie event

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Has anyone got any suggestions where I can buy something smart for a black tie event in the City? It's a joint 21st and 60th, mother and daughter , so I don't want to look frumpy for the young ones but I'm fat and don't do dresses

I'm 5,7, and an 18. Proper M&S 18.

Can't show my arms.

Would love to pull off a tux but suspect too fat. Maybe some sort of floaty jacket over wide trousers? But where does one shop for nice things in big sizes?

Budget wise maybe around £200? I'd like to find something that would do for similar smart occasions

Any ideas you knowledgeable style and beautiers?

thank you Scandi - I'd forgotten Artigiano but have had stuff from there in the distant past. Had a quick look and feeling much more optimisitc - thankyou!

let us know what you find!

Zara are calling this a sequin dress but it's really a large shirt. I think it could look well with cigarette leg trousers.