A celebration of the achievements of the most famous siblings in British advertising could never be billed as a mere anniversary party.
Oh no. The "Saatchistory" is what it's being called, a monumental bash to mark the contribution that Maurice and Charles Saatchi have made to British cultural life over the past four decades.
It promises to be quite an occasion. The grandees of the Conservative party have been invited, including David Cameron and Baroness Thatcher – whom the brothers propelled to power with their "Labour Isn't Working" poster in 1979 – and Lord Heseltine, who was a mentor to Maurice early in his career.
Also on the guest list for the party at the Saatchi Gallery in Chelsea, west London, are famous names from the creative industries, such as Lord Puttnam, Sir Roger Moore, Bono, Sir Elton John and Sir David Attenborough, as well as Charles' friends from the arts world, such as Tracey Emin, and titans of British advertising, such as Sir John Hegarty.
But one particularly important guest could be missing – and that's Charles himself. The famously reclusive art collector was known to hide away from clients when they visited Saatchi & Saatchi's offices and avoids exhibition launches at his art galleries.
Lord Bell, who founded Saatchi & Saatchi with the brothers and who now presides over his own communications empire, had doubts about the elder Saatchi's attendance. "If he's not going then why should I? I don't happen to like nostalgia very much. I'm more interested in the future than the past. I will probably pop in for half an hour," he said. "They're all desperate for me to come but if it's being held at the Saatchi Gallery and Charles Saatchi can't be bothered to come, why should I bother? I keep asking whether he will come or not and they keep saying they don't know. I know what 'don't know' means with Charles – it means 'no'."
But others close to the event, which will take place on 9 September, say that Charles, who is married to the television chef Nigella Lawson but shuns the media, is more involved in the event than he might wish to be known. One said: "In public he says he's not at all interested in advertising but behind it he's micromanaging the whole thing, he's all over it like a rash. He probably wants everybody to come and say, 'What an agency! What a gallery!'"
The party will take place against a backdrop of the exhibition Newspeak: British Art Now, which is thought by organisers to be appropriate given that the advertising associated with the Saatchi brothers has often made the news. The art will be accompanied by a retrospective display of creative work produced over the past four decades by Saatchi & Saatchi and M&C Saatchi, the operation the brothers set up in 1995 after they were forced out of their original venture.
From the groundbreaking 1970 "Pregnant Man" poster, with the question "Would you be more careful if it was you that got pregnant?" through to M&C Saatchi's hatchet job on Gordon Brown before the last election, the creative work has been as distinctive as the family's Iraqi Jewish surname.
In spite of his reluctance to attend the party, Lord Bell is lavish in his praise of the brothers' achievements. "They have consistently produced communications work which changed the marketplace, captured people's imagination and made a real difference," he said. "They turned advertising from a cottage industry into a proper economic sector and made the world take advertising seriously."
Sir John Hegarty, who made his name as an art director at Saatchi & Saatchi before founding Bartle Bogle Hegarty, said that the brothers brought advertising into a modern era. "The Swinging Sixties didn't happen in advertising until the 1970s, with Saatchi & Saatchi leading the charge."
The party will be attended by around 1,500 guests, who have been told to "dress as you like". Saatchi & Saatchi, which is now part of the French-owned Publicis Groupe, recently worked for the Labour party on its election campaign, and senior Labour figures are also understood to have been invited.
Robert Senior, chief executive of Saatchi & Saatchi, said that through their "ambition and fearlessness" the brothers had "defined the industry both here and abroad". He said: "That has to be commended and celebrated." And no doubt it will be at the Saatchi Gallery next month, whether or not Charles deigns to appear.
Famous for being Baroness Thatcher's favourite PR man, Tim Bell is also, through his Chime Communications network, one of the most powerful and most recognisable figures in the British media. He was one of the founders of Saatchi & Saatchi in 1970, becoming the agency's first managing director.
Sir Martin Sorrell
Founder and chief executive of WPP, the world's leading advertising and communications group. He joined Saatchi & Saatchi in 1977 and was group finance director for seven years, before building his own media empire on the back of a company called Wire Plastic Products.
Sir John Hegarty
Brilliant art director who teamed with Charles Saatchi in his first copywriting job and became one of Saatchi & Saatchi's first recruits. Now one of the most famous figures in British advertising, having founded the Bartle Bogle Hegarty agency in 1982.
Designed the Pregnant Man ad for the Health Educational Council in 1970. Sinclair, sometimes known as the "Third Brother", co-founded Saatchi & Saatchi and is chairman of M&C Saatchi. Invented the "Labour Isn't Working" poster in 1979 and helped the Tories toughen up their advertising at the last election.