Dai continued: “You may recall that following the opening credits two sentences were flashed upon the screen explaining what the film was all about: ‘A civilisation vanished overnight. Everything gone with the wind.’ That’s how I feel about the Clermont Club and the Lucky Lucan episode.

“It wasn’t just the murder of the nanny Sandra Rivett and what did or didn’t happen to Lucky afterwards. I knew him, of course. Quite well. Used to play backgammon with him. Good backgammon player, but I can’t say I liked him. Dull dog. Drank too much, but that was not the point. Nor was it whether he had or hadn’t killed the unfortunate nanny.

“For me what counted was that from the moment of the murder, everything that had made the Clermont Club unique vanished. Not just the gambling but the people, and a way of life, all suddenly swept away. I remember coming here in the afternoon after it occurred. The murder was already making front-page headlines in the early editions of the evening papers, and this whole place, which was normally buzzing with people after lunch, was empty as a sinking ship. Few returned. A society, and a very interesting one, had gone with the wind.”

When all this happened back in November 1974, the handsome young Dai Llewellyn — baronet son of the Olympic champion showjumper Sir Harry Llewellyn and brother of Roddy Llewellyn, Princess Margaret’s lover — was social secretary of the Clermont Club in Berkeley Square.

The Clermont was the creation of one of the most extraordinary characters of the 1960s, John Aspinall, high priest of gamblers, showman of genius, and close friend of tigers and gorillas. He became the self-appointed Pied Piper of gambling to the English upper classes.

With high-stakes gambling at the Clermont upstairs and disco dancing in the cellars — where Mark Birley opened a nightclub named after his wife Annabel — 44 Berkeley Square was at the forefront of Swinging London.

Aspinall and his closest circle — the so-called Clermont set — lived by taking risks and shared a code of loyalty to each other. It seemed as if through gambling they had attained everything they wanted — the smartest friends, the most beautiful women. Some enjoyed enormous luck while others met with disasters. The most disastrous gamble was that embarked on by Lucky Lucan.

On November 7, 1974, somebody smashed in the skull of a 29-year-old nanny, Sandra Rivett, with a 2ft length of lead piping in the basement of a house in Belgravia where Lucan’s estranged wife Veronica lived with their three children.

Although no one was ever put on trial, the coroner named Richard John Bingham, 7th Earl of Lucan, as the murderer. But Lucan had disappeared and in spite of tantalising “sightings” of the apparently homicidal earl as far apart as India, Mozambique and South America his whereabouts, if by some faint chance he has survived, remain as big a mystery as the Mary Celeste, the lost books of Livy or the crown jewels of Ireland.

In the midst of all the far-fetched theories that were soon appearing, some basic questions were overlooked. And that would have been that, but for the appearance on television in 2000 of a celebrated south London criminal.

The story he told got me thinking about one of the most intriguing features of 1960s London: the links between some of the more louche figures of wealthy west London and the gangsters of the East End.

This set me on a trail that led back to a key witness in the Lucan case, the last person known to have seen him alive. The widow of one of Lucan’s closest friends, she had spoken to me many times without giving a clue, but before she died last year she told me what really seems to have happened to Lucky.