(image via squaremeal)This is one of the best restaurants I've been to in London so far. I've been twice now, because the food is stunning and it's just an all-round enjoyable experience. Beautiful, delicate flavours, lovely ambience, charming staff, and a great place for an intimate dinner and a good chat. It's on Blenheim Street, just off Oxford Street near Bond Street, and is Michelin-starred.
The reviews (see the website) are pretty stellar as well.