'black tie' - what can I wear?

[pinksky]

I have a black tie dinner next month at St James' palace and am feeling a bit unsure about what to wear. I was thinking of wearing a dress I have worn as a wedding guest (knee length and floral) but given the venue I wonder if something more traditionally black tie (full length/cocktail) is needed.
I'm not sure where to look if so, this was the only thing that grabbed me when I had a quick search www.houseoffraser.co.uk/CC+Floral+lace+dress/147930530,default,pd.html?cgid=0356 but could do with reassurance before I spend hard earned cash on a new dress, or suggestions for other dresses.
Thank you

I am too common to understand the conventions of black tie, but could you google the event to find some photos of previous ones? That may give you a better idea of what people wore/will wear...

As for full length dresses- this is lovely... as is this. and both reduced.

Thank you very much, TethersEnd - the oasis one you linked to is beautiful - only size 10 left on the website which is too small for me, but I may call and see if they have one in store.
I have googled with little success, but am guessing it's better to be conservative. I know two people who are also going, but they are both men in their 60s, and haven't been much help!

Having been to a few black tie events in military messes, I think either the dress you've got or the Oasis one will be fine. I'd probably steer clear of the blue one unless you are attending for purely social reasons, and not professional ones like receiving an award.

Fantastic, thanks Meow - yes, it's professional so keen to look right.

You're welcome