I can recommend the following restaurants in Central London, mainly Mayfair area. They are all fairly top-end, so none of them are by any means cheap, but all of them offer excellent food, and are nice and sedate! (though they can be a little formal)
Bentleys (one of London's top fish restaurants): http://www.squaremeal.co.uk/venues/london/view/23906/Bentley's
Racine (great French restaurant/brasserie in Kensington/Knightsbridge area)
http://www.squaremeal.co.uk/restaura...w/81247/Racine
Alloro (upmarket Italian in Mayfair)
http://www.squaremeal.co.uk/restaura...w/80114/Alloro
Sartoria (Conran Italian just off Regent St)
http://www.squaremeal.co.uk/restaura...81175/Sartoria
Latium (excellent Italian just off Oxford St north of Soho)
http://www.squaremeal.co.uk/restaura...w/82579/Latium
Hibiscus (formal Michelin star dining):
http://www.squaremeal.co.uk/restaura...84697/Hibiscus
The Square (formal Michelin star dining)
http://www.squaremeal.co.uk/restaura...w/81255/Square
Crazy Bear (upmarket Thai north of Soho as an alternative to Patara in Soho which is also very good as already recommended)
http://www.squaremeal.co.uk/restaura...958/Crazy_Bear
Orrery (Conran in Marylebone)
http://www.squaremeal.co.uk/restaura...w/80981/Orrery
and my absolute favourite, but exteremely expensive:
Petrus in Knightsbridge (2 Michelin stars)
http://www.squaremeal.co.uk/restaura...w/81027/Petrus
Obviously there are a lot of cheaper places but I don't think you'll be disappointed with the food at any of the above (or the ambience), as long as you don't need to watch the bill too closely!