Among the famous faces sipping champagne and nibbling canapes, a group of shadowy figures circulated, making polite conversation without once revealing their identity.
It was not an MI6 recruiting drive, but the 150th anniversary party for Who's Who and the anonymous characters were its compilers. Their identity remains a closely guarded secret to prevent bribes or pleas for inclusion. Should you ring the office you will not be told the identity of the person to whom you are speaking. Should you meet them at a party, they will tell you only that they work in publishing. If pressed, they will add only that they publish reference books.
But they were there, along with 235 people who are among the 30,000 entries in the latest edition. Among those who accepted an invitation were Lord Menuhin, the youngest person ever to be included at 15; Ben Elton; Germaine Greer; and Dame Barbara Cartland, who has the longest entry, of 221 lines.
"We couldn't ask them all so we went for people known to us, people who are fun," said Charlotte Burrows, a spokeswoman for publishers A&C Black. "There were a lot of people from the media, the literary world and publishing, but not all of them, and we thought it was an interesting mix of people."
An invitation to join the ranks of those listed in Who's Who is viewed as a sign that one has "arrived", but every year there is fierce criticism about those who have been left out. No one knows why some people are chosen and others not - the methods of selection, like the selectors, are secret. Cilla Black, for instance, is in; Sting is not. Will Carling is in; Gazza is not. Nor is Mick Jagger - but Ms Burrows would like to set the record straight. "In the early Seventies his agent was badgering us to include the band but the editor said they might all be barrow boys by the following year and wanted to wait a while.
"Then the agent arrived at the office with a barrowful of roses but his pleas were ignored. A few years later invitations were extended to the band - as they have been every year since - but Mick has ignored them."
Other notable "refuseniks" include the Barclay twins and Tiny Rowland. But for those who do accept the invitation, their autobiographical entries make fascinating reading. George Bernard Shaw listed his recreations as cycling, showing off and anything except sport. Cecil Rhodes penned an enthusiastic account of his many pastimes including "rides daily for two hours at 6am, reads chiefly the classics, of which he has a fine collection ... visits his lions every day when he can."
In the current edition the author Douglas Adams professes to spend his free time staring out of the window, and Stephen Kramer QC opts for "paying the telephone bills for two teenagers".
There is also the delicate question of age. Actors are, not surprisingly, the worst offenders. Susan Hampshire gives her birthday as 12 May 1942 but her birth certificate reveals that she was actually born in 1937. Nanette Newman and Ken Dodd both lop four years off their respective1935 and 1927 birthdays and even Mohamed Al-Fayed is surprisingly coy; he was born in 1929, not 1933.
And what of those who are simply left out year after year? Clearly Victoria Adams is Not Posh Enough Spice.