How fancy? There is a lovely vegetarian restaurant called "Mildreds" in Soho which I haven't been to in year but it's more of lovely lunch place than fancy night out. Otherwise as a complete contrast, have you been to "The Palm" in Mayfair? It's an American brand steakhouse that opened last year and does fantastic steaks if you're a meat lover. "Bank Westminster" is another I have been to a few times and liked -quite hidden away but when you walk through past the very long bar it opens up to a huge conservatory which looks over a courtyard with a fountain. The food is standard "restaurant grub" but the service is great and it's a lovely location.
Champor Champor in London Bridge is amazing - but you must book in advance.
The Lobster Pot in Kennington does a fantastic line in seafood and has a most interesting ambience... run by a total nutter of a Frenchman. Took DH there for his birthday one year and loved it - recommend the tasting menus.
Patterson's Restaurant in Mayfair, is also very nice.
I use Toptable a lot and have found lots of good places that wouldn't normally be on my radar by using their special offers.
Gauthier Soho is AMAZINGhttp://gauthiersoho.co.uk/ and very good value in my opinion. It got a Michelin star this year, within a year of opening, but is still quirky/informal at the same time. Without doubt my favourite London reastaurant at the moment. If you're a meat-lover, St John or St John Bread and Wine (its brasserie sister ) http://www.stjohnrestaurant.com/ are both superb too, and has some very English-English old-fashioned dishes (if that's something you'd like to experience).