The prime minister was Harold Macmillan; his wife was Lady Dorothy, rooted by birth in the English aristocracy, and her lover was Bob Boothby, later ennobled by Macmillan as Baron Boothby of Buchan and Rattray Head.
The affair ended only with Dorothy's death in 1966. The fact that it never became public was a tribute to the docility and decorum of the press and to the ability of politicians and society to close ranks against outside scrutiny. In any case, these were far more modest times. Sex was not yet openly discussed - not even between husband and wife - and to splash details of illicit affairs would probably have been counter-productive. It is tempting to conclude that those were more civilised times.
Harold Macmillan, who was prime minister from 1957 to 1963, believed in fidelity, loved his wife, and was heartbroken when she died. He behaved immaculately throughout her long affair, giving his name to Sarah, her daughter born in 1930, fathered by Boothby. Much later on he treated the troubled and unhappy young woman with great kindness. Contemporaries have described Macmillan as 'a cold and unfeeling man, especially where sex was concerned'. This may have been true, but nothing can detract from his generosity to Sarah, whose paternity was never in doubt.
Lady Dorothy Cavendish, third daughter of the ninth Duke of Devonshire, was born in 1900 and brought up in the old tradition of great houses, nannies, governesses and noblesse oblige. She met Macmillan in 1919, when he was aide-de- camp to her father, then Governor- General of Canada. Within months they were engaged. For an ambitious young man with political leanings (he became an MP in 1924), the connection was advantageous. He liked to say: 'I have it both ways: my grandfather was a crofter, my wife's father a Duke.'
For the first couple of years the marriage appeared happy, but before long Dorothy's high spirits and warm but turbulent nature looked for greater fulfilment than her devoted husband could offer. Richard Davenport-Hines, biographer of the Macmillans, says: 'Like many other men whose lives have got too closely entangled with their mothers', Harold was frustrated: where he loved he could not sexually desire, and where he desired he could not love.' Despite this, three children were born to them in the first five years. Then, in 1929, Dorothy met the raffish and sexually dynamic Boothby, already a promising young Tory politician.
She was captivated by Boothby's charm and sophistication; he was flattered by her attentions, which quickly developed into an overwhelming and lifelong obsession. Boothby provided fun and glamour as well as sexual fulfilment, and for the first five years of their relationship they virtually lived together. But Macmillan would not give his wife the divorce she and her lover both craved. He loved her - and in any case, divorce was unthinkable for both family and political reasons.
Davenport-Hines has studied the events of those years. He says: 'These relationships were recognised in the past for what they were - an affair of passion - but passions have gone out of life now, and been reduced to sex, while journalists behave like children trying to burst into their parents' bedroom. Passion can be a higher form of sensibility, and it was admired as such, but it can only flourish amid tension and obstacles. The Boothby/Lady Dorothy affair was a magnificent passion based on obstacles: and if they weren't there, she created them. Obstacles made for desperation and excitement. It is impossible to avoid the conclusion that she actively enjoyed scenes and melodrama.'
Extraordinarily, in his autobiography, Recollections of a Rebel, published 12 years after Dorothy's death and 11 years after his marriage to a woman 33 years his junior, Boothby does not mention the affair at all. His mistress figures neither in the index nor the book, though this probably sprang from discretion rather than bitterness.
In 1933 Boothby wrote about Dorothy to his friend John Strachey: 'The most formidable thing in the world - a possessive, single- track woman. She wants me, completely, and she wants my children, and she wants practically nothing else. At every crucial moment she acts instinctively and overwhelmingly . . . I am passionately in love with her. But if I take her, it's goodbye to everything else.'
Dorothy did her best to persuade her lover that the world would be well lost for her sake; but Boothby's political career would have been wrecked by a divorce and his means did not allow him to support her in anything like the style she took for granted. While the establishment would protect its own - as it did the King and Wallis Simpson - it did not forgive those who publicly breached the unwritten code.
Boothby made several attempts to escape from Dorothy but his mistress's overwhelming jealousy, as well as his love for her, always prevented him. After her death he told a biographer of Macmillan: 'She was the most selfish and possessive woman I have ever known. Once, when I got engaged to an American heiress, she pursued me from Chatsworth to Paris and from Paris to Lisbon. But we loved each other, and there is really nothing you can do about this, except die. Wagner was right.' The fact that Boothby liked and respected Macmillan, and that both were MPs, made the situation worse. Members of their families, even the Conservative Party whips, took sides. Nothing short of renunciation could have restored Boothby's political hopes, and even without Dorothy he had committed plenty of other improprieties.
In 1935, believing that the affair with Dorothy was on the wane, Boothby proposed to one of her cousins, Diana Cavendish. They were briefly and disastrously married; a marriage that left Boothby feeling guilty for the rest of his life. He said: 'It is impossible to be happily married when you love someone else.' There was nothing for it but divorce: a grave step in those days. Boothby wrote to his friend Beaverbrook: 'Don't let your boys hunt me down.' The hounds of the press were duly kept on the leash.
Time passed, the physical passion between Boothby and Dorothy faded (though she continued to write letters and telephone him every day) and gradually they settled down, with Harold, into a menage a trois.
Nevertheless, the affair put an end to any hopes Boothby might have cherished of achieving high office. Dorothy's brother-in-law, James Stuart, was Tory chief whip at the time, and very much a member of the anti-Boothby camp. His disapproval handicapped Boothby's political prospects enormously. This was compounded by a financial scandal in 1941, when he was censured for not disclosing a personal interest.
The child of their tempestuous liaison, Sarah Macmillan, had an unhappy life and an early death at the age of 40. The journalist and writer Quentin Crewe recalls a lengthy relationship with her. He was an habitue of Birch Grove, the Macmillan family home near East Grinstead, Sussex, throughout the Fifties. Even then, 'Boothby used to write nearly every day, as well as telephoning most days, and Lady Dorothy would scurry downstairs first thing in the morning to snatch up the post before Macmillan saw it. Boothby was a beguiling character, of course . . . He had been a very promising young man in the Tory party, but he always had his flaws. It was the trouble over the cheque bonds in 1941 that probably sank him.
'He was a vain man, and the fact that she loved him so extravagantly was a boost to him. I remember Lady Dorothy as an odd mixture of shyness and charm and great warmth of character. It's a shame that Harold misunderstood her. He thought he had to build up the family publishing business to make himself worthy of her; he was star-struck by her. She was bored by that, and by politics, so she turned to Boothby who was flamboyant and racy and flattering. She said to me once: 'People say I'm unfaithful but I've always been faithful to Bob.'
'Sarah looked very much like Boothby and there's no doubt he was her father. She did not learn the truth about her parentage until she was about 17, when it shook her deeply. I think it was the start of her alcoholism. Once, when she was drying out in a clinic in Switzerland, Harold flew to visit her, and when she eventually married and adopted two children, he set up a Macmillan family trust fund for them.
'She was unable to have children herself as a result of an abortion the family made her go through with. This was in the late Fifties - there was a general election coming up - and people were terrified that the scandal might damage Macmillan. She did feel very bitter about that and resented it desperately.
'The whole climate has changed since then. The Boothby business was never discussed, though everyone knew about it. But it just didn't get into the papers. Barely 30 years later, everything is different - people's private attitudes to morality, and the public treatment of lapses.'
Something else has changed, according to one relative of the pair: 'People then didn't want to ruin each others' lives. The love affairs and so on went on just the same as they do today - the difference was, people didn't rat on each other. They wouldn't have dreamt of ringing up a paper: they'd have been absolutely horrified.'
For the politicians concerned, this must have been a good thing. If they were reasonably discreet, their private lives remained a matter for themselves and their immediate circle. Boothby's constituents never had to decide whether their much- loved MP was compromised by his behaviour, since it was never paraded through the tabloids.
Macmillan was prime minister at the time of the Profumo-Keeler scandal in 1963. The exposure of Profumo's flagrant infidelity must have been especially painful in view of his own situation, and it explains his outrage when the affair came to light. Yet no whisper of gossip about Dorothy ever escaped from the still tightly-knit establishment.
Many people argue that today's public gossip is indefensible. Lord Hailsham, the former Lord Chancellor, believes the law should be changed to protect people's privacy: politicians or anyone else. 'I can only suppose, without knowing anything about that particular relationship, that these considerations obtained, and I think it's more decent and more civilised. There is a moral right to privacy and I think it should be a legal right. The highest moral standards should be demanded, but if people do fall by the wayside I think their privacy should be respected. Everybody's entitled to that.'
Telephoto lenses and tape recorders mean that nobody's private life is safe, although their use may soon be restricted. Some people have protested that those in authority over us should be open to public scrutiny. But human sexuality is notoriously hard to regulate, and the fear of being found out does not guarantee faithful husbands, nor does fidelity necessarily make for happy wives.
In one respect, things today are better then they were. Now that little stigma is attached to illegitimacy, the considerations that used to limit women's sexual behaviour are no longer punitive. If Tim Yeo and Julia Stent's daughter grows up to live a happy life; if she knows her father's identity from the beginning, this - in the light of Sarah Macmillan's tragic life - is all to the good. The innocent children of ecstatic, illicit liaisons suffered in the past as much if not more than their parents. Not any longer.