I have eaten both upstairs and downstairs at L'Escargot. These 2 rooms are aimed at different clients, upstairs is more formal and expensive, downstairs, a bit less formal, menu not quite as exotic.
Nevertheless, both provide excellent dining. Downstairs, 22.50 quid (I think it is now) for a 2 course meal - great value - great flavours. I had a rich creamy lobster bisque - absolutely lovely, but I couldnt finish it all it was so flavoursome! That might sound bad to some people but I can assure you it wasn't, it showed generosity that you dont get in many restaurants today.
Similarly the beef stew (can't remember exactly what they called it), was thick, rich, with good amounts of meat and again very tasty.
Upstairs again excellent, not outright the best restaurant food in London (for me Ramsey's gourmet menu is the top), but still gets 5 stars.
Good service, good attention to detail (it annoys me when I get even slightly dirty cutlery or glasses, but no probs here). Great value, not too big (like another of MPWs places I could mention which I found v. disappointing). This place deserves your support!