Escorts in Berkeley Square, W1J, with the plane trees overhead and a black door waiting

The square was cut out of the fields behind Berkeley House in the seventeen hundreds and kept its garden while everything around it was rebuilt twice over. The planes down the middle are reckoned to be among the oldest in central London, and by July they close over the path so completely that the traffic on the far side simply vanishes. There are benches, a small pavilion and, most lunchtimes, a crowd of office workers with sandwiches who have no idea what the street does after dark.

The west side is showrooms and quiet company nameplates behind Georgian brick. The east side is where the evening lives: awnings, a fish restaurant that glitters like a jewellery counter, and the row of doors that admit members and their guests and absolutely nobody else. Bruton Street leaves the north west corner towards the Bond Street windows; Berkeley Street drops south towards Piccadilly. Both keep serving long after the kitchens elsewhere have wiped down.

Three sorts of evening land here. The first is the members' night: a guest with an invitation to one of those doors who wants company that looks entirely at home inside and never asks what the subscription costs. The second is the long supper, booked by someone staying nearby who wants dinner five minutes from the hotel and a bar afterwards that is still busy at midnight.

The third is the celebration. A birthday, a contract signed that afternoon, a final night in town before a dawn departure. The square takes all three in its stride, because everything sits inside the same two hundred metres and nothing shuts early. Bella hears one club named here more than any other, and her answer never changes: your companion will be beautifully dressed and entirely discreet, and the door itself is your side of the arrangement, not ours.

W1J 8LT holds the Stratton Street corner, with a bar that stays loud and a screening room downstairs. W1J 7BH is the quiet alternative, a run of townhouses knitted together on Half Moon Street around a small panelled bar that suits a first drink.

W1J 5EB is five minutes south on Charles Street and feels far more like a house than a hotel. W1S 4BP is about the same distance north on Albemarle Street, all fireplaces and long afternoons. W1K 2AL needs a walk through the back lanes and earns it at the bar.

We list those doors because guests ask where they are, not because we have any interest in them. Nobody on or near this square works with us, nothing about you reaches a front desk, and your companion simply walks in as your guest: a surname given at the bar, or a lift taken alone once Bella has a room number and the name it sits beside.

Green Park lies roughly 4 minutes south down Berkeley Street, where the Jubilee, the Piccadilly and the Victoria all meet. Bond Street is about 6 minutes north west through Bruton Street, bringing the Central line and the Elizabeth within reach of anyone arriving from the airport or the City. The Piccadilly line also calls at Hyde Park Corner, some 10 minutes west past the showrooms.

Travel comes with her: a car, and everything within the first two zones already counted in the figure Bella gave you. Ask for tonight and the knock usually comes about an hour after Bella confirms, later when the square's traffic is at its Friday worst, with the exact time dropped into the chat as she leaves home; ask for Friday and Friday is what you get, to the minute. Her fee passes across at the door itself, and one written exchange settles the room before she sets off.

Margo supplied this one. A guest in a suite on the Stratton Street corner, four hours booked from eight, glossy dark hair and that famous ring on the wrong finger. They opened with a table on the east side of the square, where she talked him out of the bottle he was about to order and into something Burgundian at half the price.

By ten they were walking the garden itself, the planes black overhead, and she was doing her impression of the doormen without moving a muscle in her face. The last hour went on a nightcap back at the hotel bar, where a pianist was murdering Satie, which she insists improved it. The four hours came to £1,300, and a note reached Bella next morning wanting to know when Margo has another Thursday free.

Samanta lives a few streets north and reads a room the way a violinist reads a score, which is precisely what a members' night wants. Angelika is the one to ask when the evening is heading for a basement with a band, and Luna will find the single bottle on a long list that justifies the detour.

From Knightsbridge, across Hyde Park Corner, come Margo with her late Satie habit and Sonya, whose ballet posture turns any doorway into an entrance. Violetta comes up from Belgravia with a punchline saved for the taxi home. And when the night is going to run genuinely late, Jasmin and Molly cross from Soho, where late is simply the house style.

Every corner hands you on somewhere. North west along Bruton Street the windows of Bond Street begin. North through the back lanes lies the terracotta of Mount Street. South down Berkeley Street you meet Piccadilly, and a few minutes beyond it Green Park and the hotels that face the trees. The district entire, corner to corner, is on the Mayfair page.