Artisan, at The Westbury Hotel, is one of my favourite restaurants in London. Serving seasonal modern European food, the menu changes regularly. Don't let the fact that it is located within a hotel put you off. The decor is classic, the service is impeccable and the food fabulous. And whilst everything is to a high standard, the place doesn't feel pretentious. I have been there on business lunches, and for dinner with friends.
http://www.westburymayfair.com/artisan.html