What to wear to a 5 star hotel wedding in central London?

I'v been invited to my cousin's wedding at a 5 star hotel in London in March and have no idea what to wear. I'm going to both the ceremony and the recption so will need something that will be suitable for both. The invitation states "black tie". I'm a 24 year old girl - can anyone recommend something I could wear? Bearing in mind it's usually pretty cold in London in March! Thanks!

Since it is a 5 star hotel, I am going to say this; Wear something very fancy. Everyone else probably will be wearing something fancy, so you do not want to be left out. Visit a place where you can rent something, because I don't think it would be worth buying something for a one time occasion like this. If you still don't feel comfortable, ask the person who invited you and I'm sure they can tell you.

A fancy getup, something that you would not normally wear and that shows that you have a little bit of class. But DO NOT out do the bride, or the groom. If it says "black tie", you need something not overly fluffy or ruffly, or neon colored. Go for a neutral color.