Escorts in Belgravia, SW1X and SW1W, as the Society arranges them after dark

Belgrave Square is the largest and the most official: forty something embassies and institutes behind the porticoes, flags on half of them, and a private garden in the middle that almost nobody is allowed into. Eaton Square is longer than it is square, a canyon of white terraces with a road down the middle and the plainest grand houses in London along both sides. Chester Square, smallest of the three, is where people actually seem to live.

Between them run the two streets that do the district's shopping. Elizabeth Street sells bread, flowers, hats and chocolate on a scale that suggests nobody there cooks from scratch. Motcomb Street, wider and more theatrical, has the old Pantechnicon front at one end and a handful of rooms where the district drinks before dinner. Behind everything are the mews, which are no longer stables and have not been for a century.

The guests who write from these postcodes are residents more often than visitors. A house that entertains twice a month, a flat kept for the London half of the year, an embassy dinner that needs a plus one at short notice. What they ask for is a companion who will not be the most conspicuous person in a room where conspicuousness is the only real offence.

The hotels sit around the rim of the district rather than inside it, which suits the geography. Not one of them is connected to the Society in any fashion. They are written down for orientation only, and your companion enters one of them as nobody's guest but yours.

SW1X 7RL holds the Knightsbridge corner at Wilton Place. At Hyde Park Corner itself stands SW1X 7TA, in a building that began life as a hospital. SW1X 7HJ is the newest of them, on the same corner and built to look at the park.

At the Victoria end stands SW1W 0JW, with a garden of its own off Buckingham Palace Road, still run by the family whose name is above the door. SW1W 0PS is a small house of rooms above the Royal Mews, and a larger red brick hotel shares that postcode a few doors along. Everything between those two ends is residential, and that is where most of these evenings happen.

A private house needs a little more from you than a hotel does. Cressida will ask for the street, the number, and which of the two doors it is where a terrace has been divided, since half of Eaton Square has a lower entrance and an upper one and the numbers do not always agree with the bells. Say if there is a porter, and say if you would rather she did not use the main steps.

Four stations ring the district. Victoria, five minutes from Eaton Square, gathers the District, the Circle and the Victoria lines above its main line platforms. Sloane Square is six minutes west, Hyde Park Corner seven minutes north, Knightsbridge eight. Your companion ignores all four; a car sets her down at the door you have named, and if that door is inside the first two zones the Society meets the fare.

For a dinner at eight, write by five. That gives Cressida time to hold a companion, confirm the address back to you in writing and tell you the moment she has left. The fee passes to your companion by hand once she is inside, and there is no later adjustment.

A guest hosting eight at home wrote in the afternoon. He wanted four hours on the dinner date, £1,800, and a companion who would not need the seating plan explained. Cressida sent Patricia.

She arrived at half past seven by the lower door, as asked, and spent the first twenty minutes talking to the guest who had come earliest and knew nobody. At dinner she was placed between a diplomat and a man who collects prints, and by the pudding the print collector had promised to show her something at a gallery she had named herself. The last car went at half past eleven and the host was told, twice, that his table had been the best of the season.

Lea, Violetta and Patricia all give Belgravia as home and know its doors and mews by heart. Lea notices rooms before she notices people and will tell you what is wrong with yours if invited to. Violetta is taller than anybody else on the roster and keeps the best joke of the evening back until it is worth telling. Patricia is the calmest of the three and the best at a long table.

Close by, Lucy and Roxi come up from Pimlico, and Bianka and Sonya cross from Knightsbridge. Michelle arrives from Chelsea and confirms quickly when the evening is arranged late. The whole circle comes to SW1 without complaint.

North across Hyde Park Corner lie the hotels of Park Lane, two minutes by car. West along Wilton Place is Knightsbridge, walkable in eight. North east, past the palace, the streets climb into St James's, and beyond that Piccadilly and the whole of Mayfair. For a dinner at home that runs later than planned, there is the note on a small party in a suite.