When the 1960s began to seriously swing and the youth of the day were learning to rebel, the Establishment was personified by its old Etonian Prime Minister Harold Macmillan.

At almost 70 years old, the Conservative had been the country’s leader for six years. A wealthy aristocrat, while not in London he lived with Dorothy, his wife of 40 years, at their home in the handsome Grade II-listed Birch Grove at Chelwood Gate in Sussex.

It was at this house that the Macmillans had received numerous heads of state, including American presidents Dwight D Eisenhower and John F Kennedy, five months before he was assassinated in October 1963. The French president General Charles de Gaulle, Russian leader Nikita Khrushchev, and the first prime minister of India, Jawaharlal Nehru, were all Birch Grove guests at one time.

But these were strange times indeed, for while the Macmillans appeared to lead the most respectable of lives here in Sussex, Dorothy was embroiled in a secret, 40-year-long affair with Macmillan’s friend Lord Boothby.

Dorothy’s relationship with the loud and foppish MP remained unreported until her death at Birch Grove in 1966. Reporters in the know at the time thought it their duty to keep quiet about it. But their silence on Boothby’s relations with notorious gangster Ronnie Kray was a different matter entirely.

Boothby gagged the press and hushed up politicians, delaying the Kray twins’ arrest by five years. The link between the British Establishment of the 1960s and the Krays was the greatest unreported political scandal of its time, allowing the gangsters to walk free for five years.

The story begins with prime minister Harold Macmillan, an image-driven Tory moderniser and heir to the Macmillan publishing empire. His wife Dorothy, formerly Lady Dorothy Cavendish and the daughter of the 9th Duke of Devonshire, were married in 1920. She began her affair with Bob Boothby a few years later.

Harold was deeply hurt, and wrote, “I still (for some reason or other) love her so ridiculously ... I care so desperately that the only way is to pretend to myself not to and to try to think about the gold standard and inflation and unemployment – about which I care hardly at all.”

For a rising politician, divorce was not an option, so a friend urged him to accept “an open marriage”. Boothby was pleased with this arrangement, and extraordinarily the two men remained cordial towards one another. Boothby even had the nerve to ask Macmillan for a peerage, which he was granted.

But then Boothby met Ronnie Kray. The Kray twins – brutish boxers turned gregarious gangsters – were icons. Unusually for the time, Ronnie Kray was open about his sexual preferences and after being introduced by mutual acquaintance MP Tom Driberg, enjoyed illicit encounters with the bisexual Lord Boothby.

But the Lord’s luck finally ran out.

On July 11, 1964, the Sunday Mirror splashed across its front page: “Peer and a gangster: Yard probe public men at seaside parties”. Boothby tried to ride out the allegations, but in the next week’s paper, a picture surfaced of him on a sofa with Ronnie Kray.

Labour leader Harold Wilson, eager to suppress details of Driberg’s role ahead of the election, nudged Boothby in the direction of the best lawyer in the business. He won outright and The Mirror awarded Boothby £40,000 damages for libel.

Later, a trial mysteriously cleared the Krays of blackmail, fraud and extortion despite a wealth of evidence. Most of the political elite knew very well what was going on, yet they chose to say nothing. They let a psychotic gangster and his brother walk free to pursue crime, until they were finally found guilty of murder in 1969.

This was silence most foul.