Escorts in Grosvenor Square, W1K, as the Society arranges them after dark

Laid out for the Grosvenor estate in the 1720s, the square was the grandest address in London before that century ended. Its houses have been rebuilt, thinned and rebuilt again, so the architecture no longer agrees with itself and nobody appears to mind. The garden is the constant. Franklin Roosevelt stands in bronze near its centre in a cloak, and Dwight Eisenhower keeps to the western edge, beside the building that served as the American embassy until it moved to Nine Elms.

That departure changed the evenings here. Where there were visa queues there are now hotel doors, and the square has settled into something calmer: a place where people are entertained rather than processed. Guests who write to Cressida from W1K are usually mid-visit, with four days in London, two of them already spoken for, and one evening they would rather not spend alone in a lobby. What they ask for is seldom a night out. It is company that can stand through forty minutes of introductions without visibly counting them, and then sit down and be herself.

Guests near the square stay at one of five houses. Each is a landmark only. The Society deals with no hotel, reports to no hotel and asks nothing of one; you give Cressida a door, and the woman who knocks at it that evening is there because you invited her.

The largest house stands on the south side at W1K 2HP, near enough that the garden is the view from half its rooms, and its function floors are where a good many of those autumn parties are actually held. W1K 6TF is a few minutes west on its own small square, a white Deco front with a bar small enough that a conversation can be heard in it.

W1K 4HR is four or five minutes east along Brook Street and needs no description here. W1K 2AL lies six or seven minutes south on Carlos Place, where guests come down early because of the bar rather than in spite of it. W1K 7TN faces Hyde Park at the top of Park Lane, and its Great Room is the largest ballroom in the district; if your evening is being held inside it, the Park Lane page is the better one to read.

Three stations serve the square. Bond Street is roughly four minutes on foot to the east and puts you on the Central line, the Jubilee and the Elizabeth line at a single entrance, which is how most guests reach W1 from Heathrow. Marble Arch, a Central line stop, is about six minutes north west. Green Park is nearer ten, down Davies Street and across the far square.

Your companion uses none of them. A car brings her to the door you name, and the cost of it inside Zones 1 and 2 belongs to the Society, not to you. Written to in the evening, she is generally at your door about an hour after Cressida's yes, a little more at the height of the rush or on a Friday, and the exact time follows in the thread once she is on her way. An evening arranged over lunch simply happens at the moment you chose.

Cressida wants two things: the name your room stands in and its number, or, for a private flat, the street and the bell. She writes both back to you before anyone moves, and asks for neither a second time. The fee goes from your hand to hers as she steps in, and the money is done with. The booking page sets the four steps down in order.

A guest is in London for a board meeting and a party he has been told he must attend. He writes at five: company from seven, drinks on the square until half past eight, dinner after that. By half past five he has a name, Samanta, and a figure, three hours at £1,500.

She meets him in the lobby rather than upstairs, which is the right way round. At the party she is introduced eleven times and keeps nine of the names, which is nine more than he manages. They leave without ceremony at twenty to nine, walk the four minutes to Carlos Place and eat late in a room where somebody has thought about the lighting. He is back across the square before midnight and has spent nothing on explanation.

Samanta, Angelika and Luna give Mayfair as their home district and live a few streets from the railings. Samanta is the steadiest presence the Society can send into a room of strangers. Angelika has the useful habit of asking the second question. Luna is the one to send when there is a wine list worth arguing with.

Further out, Rosa and Liana cross Oxford Street from Marylebone, while Margo and Sonya come over from Knightsbridge by Hyde Park Corner. Nancy makes the run up from Covent Garden when an evening is a late one. The rest of the circle comes to the square as readily.

Walk south from the railings and Mount Street is three or four minutes away, which is where the Society's tables are usually kept. East along Brook Street, then down Davies Street, brings you to Berkeley Square and the latest suppers on this map. West the streets give out at the traffic of Park Lane, with Hyde Park dark behind it, and the wider Mayfair page holds everything in between. For the houses on the square themselves, the note on the W1K hotels is the shorter read.