Escorts in Green Park, W1J and SW1A, as the Society arranges them after dark

The grass runs from Piccadilly down to Constitution Hill without a formal bed anywhere, a habit the park has kept for so long that its explanations are now folklore rather than record. In March the slopes turn yellow with daffodils and in high summer the deckchairs appear; the rest of the year it is trees and a path. The ironwork at the Piccadilly entrance came from Devonshire House, and the Canada Memorial lies further down, a wedge of granite with water sliding over its maple leaves, lit at night and almost always empty.

The eastern edge is the Queen's Walk, which runs from Piccadilly down past the back of Spencer House to the Mall. It is the shortest way from a hotel on this corner to St James's and, for the ten minutes it takes, the least overlooked walk in central London.

Guests who write from these addresses tend to be the ones whose plans have gone loose in a pleasant way. A deal signed that afternoon, a flight that leaves at teatime tomorrow, a room that faces trees. They ask for an evening with no scheduled end, which on this street is a reasonable thing to want.

Six doors sit within a few minutes of the park gates. No hotel here has an agreement with anybody at the Society, and none is spoken to. The names are for your bearings. A lobby sees a visitor crossing it and draws no further conclusion.

W1J 7BJ stands directly opposite the trees on Piccadilly and its apartments next door suit a stay of several days. W1J 7BH is a minute behind it on Half Moon Street, a terrace of Georgian houses joined into one hotel. W1J 8LT sits three minutes north up Stratton Street and keeps a lobby busy late enough that nobody looks up.

Across the Piccadilly traffic, SW1A 1NJ and SW1A 1NY hide at the ends of two quiet cul de sacs, four or five minutes away; the St James's page covers that side properly. W1J 9BR sits on the corner where the park meets the street, and belongs as much to Piccadilly as to the park.

Green Park station is a minute from the gates and puts three lines under one roof, the Victoria, the Jubilee and the Piccadilly, which between them reach Heathrow, both Bond Street and Oxford Circus, and Victoria for the Gatwick trains. Hyde Park Corner, a Piccadilly line stop, is about six minutes west along the railings. Piccadilly Circus is eight minutes east.

Your companion arrives by car, and where the address falls inside the first two zones that journey is ours. The park side is where the Society's latest arrangements are made: Cressida reads messages through the night, and a request sent at midnight is usually answered within a few minutes, and even with the streets empty, your companion is reckoned at about an hour from Cressida's confirmation, with the exact time written into the thread as she leaves.

For an overnight, ten hours at £3,000, she likes a little more notice, ideally the same afternoon, so that your companion can bring what a morning needs. Cressida writes down whose name the reservation carries and which floor and number, repeats it back, and stops there. She is given the fee once she is inside, by you, and there is nothing further to settle.

A guest with two more days in London wrote at half past ten from a room on the fourth floor facing the trees. He wanted an overnight, and Cressida had Milena at the door before midnight.

They had a drink downstairs where the pianist was finishing, then a second upstairs with the curtains open and the park black below. She slept on the side nearer the window because she asked to. In the morning they took coffee at a table by the glass, watched a man walk two spaniels along the diagonal path, and she left at ten, which was twenty minutes later than ten hours allowed and nobody mentioned it.

Angelika, Luna and Samanta live in Mayfair, just north of the park, and are the closest homes to this corner. Milena says yes to an overnight as readily as to a dinner and comes in from South Kensington. Violetta, who stands taller than anyone else the Society introduces, crosses from Belgravia through Hyde Park Corner.

Lucy comes up from Pimlico and can name what is overhead if the sky is clear from your window. Michelle travels from Chelsea and confirms faster than almost anyone. Nora is the one to ask for on the evening you land, when supper matters more than conversation.

North of the gates, Berkeley Street climbs four minutes to Berkeley Square. East along the road you are among the hotels of Piccadilly at once. South across the traffic lies St James's, and west the railings run to Hyde Park Corner and the hotels of Park Lane. For rooms that let you reach a lift without crossing a lobby, there is the note on private entrances.