The formal plus one: galas, client dinners and receptions, attended well

When you write, give Octavia the essentials. The venue, the dress code, the hour the doors open and the hour you expect to leave, and how you would like your companion introduced; a first name and the words a friend are the usual form. Mention anyone at the table she ought to know about, and any subject best left alone. Octavia passes all of it on, so that she walks in already knowing the room.

A good share of London's formal calendar takes place within a few streets of St James's. The Royal Automobile Club and the Institute of Directors both stand on Pall Mall. Spencer House looks across Green Park. The Royal Academy at Burlington House lends its rooms for dinners after the galleries close, and the auction houses of King Street and New Bond Street hold evening sales with a crowd of their own. In Mayfair, the ballrooms of Grosvenor House and The Dorchester carry the larger charity nights.

The notebook has more on discretion with drivers and the third person, and on the formality of an evening at the Ritz a few doors along Piccadilly.

The art world keeps a formal diary of its own, and much of it happens here. Frieze week in October fills the Mayfair galleries with evening openings. The evening sales at Christie's on King Street draw black cars two deep along the kerb. The dealers of Mason's Yard and Duke Street St James's hold private views most weeks of the year. Margo is the obvious companion for a saleroom: she can read a sale catalogue cover to cover for pleasure, and she knows when to keep her hands in her lap.

Once at the table, she takes her cue from you. She is genuinely interested in the people either side of her, remembers names after hearing them once, and can tell when a conversation between two guests has turned to business and is better left to them. At formal dinners she drinks sparingly. She never steers the talk towards herself, nor towards how the evening was arranged.

Some formal evenings leave town: a dinner at a country house, a ball in the Home Counties. These are arranged in the same way, with the taxi fare beyond Zone 2 quoted beside the fee.

Formal evenings overrun. Three hours at £1,500 is the usual choice for a reception followed by dinner, and the four-hour line at £1,800 for a seated gala with speeches. A drinks reception alone fits two hours at £1,100. If the auctioneer is still on his feet at eleven, extending is confirmed in writing with Octavia, provided your companion's diary allows. Every line is on the rates page.

Karolina, tall and upright, plans ahead and brings a pianist's patience to a long evening. Margo knows the salerooms better than many of the people bidding in them, and she never says so. Sonya, calm and precise, with a dancer's carriage, is ideal for an early dinner that ends by eleven. Violetta is Octavia's choice for a long table of people you need to impress. Three of the four are in their thirties, which some hosts prefer at a formal table.

Tell Octavia at the outset if your face or your name is familiar to the public. Your companion takes no photographs, posts nothing, and on any later occasion leaves it to you whether the two of you have met before. The details of the evening remain in the thread in which it was arranged. Booking explains the single check made beforehand, and why it goes no further.