Escorts in St James's, SW1A and SW1Y, as the Society arranges them after dark

Jermyn Street runs parallel to Piccadilly and sells shirts, shoes and scent from premises whose fronts have not been redesigned in living memory. The street called St James's runs uphill from the palace gate to Piccadilly, a hatter on one side and a wine merchant on the other, both still weighing and measuring in rooms with sloping floors. King Street holds a saleroom whose porters carry things across the pavement that are worth more than the building.

St James's Square is the centre of it: a garden, a bronze king on horseback, and a library on the north west corner whose members climb iron stairs to reach their books. Around the square the plate glass is offices now, but the doors are the old ones, and at seven in the evening the square empties in about four minutes.

The guests who write to Cressida from SW1A and SW1Y are the quarter's own: bankers, dealers, people who have come to London for two meetings and a dinner. They rarely want a late night. They want an early table, a companion who can hold a conversation in a room of eight, and to be back at the hotel before the streets have finished emptying.

The hotels here are built to a different scale from the Mayfair houses, which is much of their appeal. These four are landmarks, nothing more; the Society deals with you and not with them. Say which one and Cressida knows the door.

SW1A 1NJ hides where a short cul de sac leaves St James's Place. Club ties and signed photographs cover its bar to the ceiling, and a mews of converted stables stands behind. SW1A 1NY is a minute away at the end of another closed street, and its martini is poured at the table from a trolley, which is either the best or the most theatrical thing in SW1 depending on the evening.

SW1Y 6JF stands on Jermyn Street, larger and easier and useful when everything else is full. SW1Y 4AN fills a banking hall at the Waterloo Place corner, five minutes east. W1J 9BR keeps the northern corner of the quarter, and is treated on the Piccadilly page.

Pall Mall is a row of clubs, and a club is a private house with rules. Where you hold a membership, or are being signed in by somebody who does, a companion comes with you on the same footing as any other guest, dressed to whatever the club expects and introduced by name. What the Society cannot do is get anybody through a door that is not theirs to open. Cressida says this plainly at the outset, instead of leaving it to be discovered at a porter's desk.

Where a club is not the plan, the quarter is well supplied with alternatives. The bars at SW1A 1NJ and SW1A 1NY are open to anyone with a reservation, several dining rooms on Duke Street and Bury Street keep small panelled rooms for four or six, and an early table in St James's is easier to get than a late one anywhere else in W1.

Green Park station is three minutes uphill and carries the Victoria, Jubilee and Piccadilly lines. Five minutes north east, Piccadilly Circus adds the Bakerloo. St James's Park, six minutes south west past the palace, is a Circle and District station.

Your companion comes by car, and a journey ending inside the first two zones is settled by the Society. The quarter's rhythm is early, so Cressida likes to hear from you by six for a dinner at eight, which gives her time to hold a companion and a table in one exchange. She sets down in writing which room, held in whose name, and asks nothing further; what she is owed passes from you to her directly, at the door.

For a dinner of six or eight where you would rather arrive together, say so and your companion will join you at the hotel half an hour beforehand. The booking page sets the steps out plainly.

A guest staying on St James's Place wrote at four: two colleagues and their spouses at eight, and he did not wish to be the one man at the table without company. Cressida proposed Karolina and four hours over dinner, £1,800, so that nothing would need to be watched.

They met at half past six at the trolley. She had the barman explain the lemon peel to her and asked one question about it that he clearly enjoyed. At dinner she talked to the quietest of the six for most of the second course and left him convinced he had done the talking. They walked back across the square at half past ten, past a building with one lit window on the second floor.

Karolina comes over from Covent Garden and is at her most assured where nobody raises a voice. Samanta and Angelika live just over Piccadilly in Mayfair. Sonya crosses from Knightsbridge and keeps early evenings, which suits this quarter exactly.

Patricia and Violetta come from Belgravia by way of the park. Liana travels down from Marylebone for a dinner worth the journey, and Lucy comes up from Pimlico. Every companion on the roster will come to SW1.

North across Piccadilly you are beside Green Park in three minutes and among the hotels of Piccadilly at once. Keep going up Berkeley Street and Berkeley Square is five minutes away, with the rest of Mayfair beyond it. Eastward the quarter thins into Haymarket and then the theatres. For the rules that govern a guest signed in at a club door, there is a note on members' clubs.