For some time there had been rumours that the Snowdons could 'no longer support life together'. When the announcement was made, the Palace tried to soften its impact by saying that there were no plans for divorce. Even as it stood, however, the news destroyed the conventional image of royalty. Lord Harewood, though royal, had been little known to the public - indeed, barely at all, until his marriage problems gave him a fleeting prominence. Princess Margaret was somebody people had grown up with. Although marriage break-ups were, by now, ten-a-penny in the outside world, the revelation that the perfect family suffered from the same tensions as many imperfect ones came as a shock.

Who was responsible? The question dominated articles and features on the subject. Quality newspapers blamed popular ones. 'Almost since the day of their marriage the press, fed by bitchy society gossip, took a prurient and intrusive interest in their private life,' noted Philip Howard. The Queen's private life, he suggested, was 'generally considered inviolable by such malice,' as well as being too worthy and dull for it. But the Princess was reckoned fair game 'for our national hypocrisy masquerading as morality'. Howard's comment contained an echo of Malcolm Muggeridge's, almost twenty years before. 'Each time that the Princess acquires a new escort the gossip writers set to with a will,' wrote the pioneering castigator of the royalty industry, in the aftermath of the Townsend affair. 'Once more the cameras click, the typewriters tap, the wires hum...' But even such 'responsible' comments, from respected journalists, came out of a gramophone groove. Making copy out of the hypocrisy of others who made copy out of Princess Margaret's embittered love life (or out of any other royal gossip) was a long-standing stratagem.

Cameras had clicked and type-writers hummed throughout the Snowdon marriage. The couple guarded an area of privacy, and Snowdon continued a professional life which gave him a status in the outside world that was unique among those who had married members of the Royal Family. However, the Snowdon set, involving show business personalities who were often indiscreet, provided a steady flow of stories. Rumours of marital trouble began to appear as early as 1967. 'The problem was two large egos,' says a friend. 'Both were rather spoilt. They competed with each other in a funny way. They were dotty about each other in the beginning, but they were only really happy for about two years.'

Snowdon had formed a relaxed, friendly relationship with his sister-in-law, and with other members of the Royal Family. In their early married life together he and his wife had thrown themselves into joint enterprises, like the re-organization and redecoration of their apartment at Kensington Palace. However, Snowdon - with no background of royalty, and with ambivalent feelings about what being royal involved - became increasingly frustrated by the restraints the status entailed, and by the ersatz fame. 'Part of the problem was that this ghastly infidelity crept in, which is always corrosive to a relationship,' according to one source. 'He used to humiliate her in public, she nagged and got peeved. He behaved badly - he used to do things like leaving early at dinner. They used to fight and shout when it went wrong. She used to say "I can't talk to him, he won't listen".' There was press discussion about a 'structural fault' in the marriage, and about 'clashes of temperament, reflected occasionally in manifest and semi-public rows'. Part of the structural fault was that Snowdon wanted to pursue his independent, non-royal career and found that despite the initial advantages, being royal got in the way. Eventually neither he nor, in the end, his wife was prepared - for the sake of duty, patriotism, or Church dogma - to live out the marriage as a sham, once it had become irretrievable. It was agreed that the two children - Lord Linley, aged fourteen, and Lady Sarah Armstrong-Jones, aged eleven - should stay with their mother.

The Monarch kept out of it. 'There has been no pressure from the Queen,' stressed Allison, her press secretary, 'on either Princess Margaret or Lord Snowdon to take any particular course.' The break-up was conducted with dignity, and lack of recrimination. Nevertheless, it was a double blow to the Monarchy as an institution. Princess Margaret was remembered both for her dutiful renunciation of Peter Townsend, and - by older people - as a member of the 1930s and 1940s quadrilateral Family which had stood as the Empire's ideal. The effective end of her marriage cracked the glass, throwing doubt on comforting images of the past, as well as of the present. At the same time, it carried the savage message that the Archbishop, the editor of The Times, the Cabinet and other moralizers who had advised the Queen about Peter Townsend's suit had, in human terms, been wrong; and that the Queen might have done better to have stood up to them.

As far as the popular press was concerned, the separation was as much a beginning, as an ending. If Margaret, now forty-five, had decided to become a nun (Prince Philip's mother had made such a decision, under similar circumstances), the result might have been different. However, it was not in her nature. The press was quick to sense it. 'When my sister and I were growing up, she was made out to be the goody-goody one,' Margaret was later quoted as saying. 'That was boring, so the Press tried to make out I was wicked as hell.' Alongside the semi-canonized little Princess, there had always been - in the eyes of professional observers - an alter ego, the naughty minx, the willful younger sister. Reports of the separation, though couched in sepulchral tones, referred elliptically to Roderick Llewellyn, a twenty-eight-year old trainee landscape gardener.

The references were not new. For some time the adventures of the Snowdons, hinted at in the popular papers, had been honing down the conventions which had hitherto made taboo all references to royal entanglements. Margaret's matrimonial difficulties, and her friend ships, became the highway to a new, more raucous kind of press voyeurism, which combined the snobbish, the coy, and the explicitly sexual - encouraged by a new generation of less gentlemanly editors, and by competition between the faltering middle-market Daily Mail and Daily Express. The Mail's success in using a more aggressive style of royal reporting had incited the Express at about the time of the Snowdon separation to follow suit. It did not take long for the big-selling tabloids to join in. Rumours became blazoned scandals, and what was left of the old sanctimonious reticence was transformed into competitive ribaldry, couched in the language of homily.

The story of the last days of the marriage tumbled out. Nigel Dempster, doyen of the gossip columnists, described how 'Margaret and Tony blithely seemed to tolerate each other's liaisons' - until the appearance of Roddy Llewellyn and of Lucy Lindsay-Hogg, the young woman Snowdon eventually married. A paparazzi picture in the News of the World of Margaret and Llewellyn sitting at a table in beach clothes in Mustique, where Margaret owned a house, had apparently precipitated Snowdon's departure from Kensington Palace.

After the split, Snowdon's private life was, to some extent, left alone. The same was not true of Princess Margaret's, and the supposed incongruity of her relationship with a man much younger than her who relished the publicity his affair had given him, was irresistible to the media. As soon as the separation was announced the press laid siege to Llewellyn, who accepted £6,ooo from the Daily Express for arranging a photograph of his friends. From Buckingham Palace's point of view, the seven-year Roddy affair became a public relations nightmare - even though other members of the Royal Family seldom if ever met him, and only knew of his activities from the press. From the media viewpoint, however, Llewellyn's meetings and holidays with Princess Margaret provided a rich seam of the scandalous, the scurrilous and the absurd.

When people dreamt about the Royal Family, as one journalist put it a few years later, 'women dream that the Queen comes to tea - and men dream that Princess Margaret falls in love with them'. Margaret's unusual friendship was the stuff of fantasies. It was also the stuff of music-hall jokes. Soon, 'Roddy for PM' tee-shirts were for sale in London boutiques, and royal frolics became daily tabloid entertainment, with a mixed message - the allegedly dissipated life style of the idle rich, the embarrassment to the Queen, and the squandering of Civil List money. In April 1978, an ORC poll showed that seventy-three per cent of the public thought that Margaret had damaged her standing as a member of the Royal Family. She had been a butt of press attacks before the separation. Now she was pilloried in a way that had never happened to a member of the Royal Family in the twentieth century. Critics noted that her public engagements had halved between 1977 and 1978, and that a large amount of her time was spent in the Caribbean. MPs spoke of letters from their constituents, clerics of the moral example. 'Princess Margaret's personal life cannot remain purely a family matter,' said Dr Graham Leonard, Bishop of Truro, in a sermon, 'because what the Monarchy does affects society.' Dr Mervyn Stockwood, Bishop of Southwark, however, declared - with greater Christian charity- that recognizing the Princess's record of service to the Church was far more important than censoring her private life.

Contrary to the impression given in the press, Princess Margaret did not go to pieces. Indeed, she even seemed at times to revel in the attacks, as if, at long last, she was able to get her own back on the Establishment that had destroyed her happiness more than twenty years before. Though distressed by the collapse of her marriage, she was relieved when it was over, and enjoyed her husband's replacement. 'Roddy was like Tony without the vice,' says a friend. 'Tony was amazingly entertaining, but very wicked about getting at people in public. Roddy, by contrast, was a sweet character, very easy and giggly.' She did little to conceal her relationship - or to encourage Llewellyn, who happily capitalized on it, to do so either. 'There is no mystique about Mustique,' as the Mirror inevitably put it. 'Every time she holidays there with Roddy it is in the full public glare.' Instead of fending off the paparazzi, she now pretended they did not exist. She also became defiantly rude to dignitaries if they failed to amuse her. 'I did not find her an easy person to talk to,' Paul Martin, the monarchophile Canadian High Commissioner recorded after sitting next to her at dinner in May 1977. His wife, he noted, 'thinks she is quite sad and perhaps that is the reason'.

Sad, glad, or espiegle, she gave splendid fuel to the Monarchy's opponents, as the Privy Purse prepared for yet another delicate negotiation over the Civil List, and the nation faced a new crisis over pay norms. William Hamilton had never had so much attention. Soon, he and Roddy became tabloid sparring partners. 'There would be a marvellous response from working people if she were told that she could get on with her private life and that the Government couldn't go on supporting her,' said Hamilton, at the height of Llewellyn's notoriety. 'I would like to see Willie Hamilton . . . do all the jobs in the marvellous way she does,' Roddy gallantly but unwisely responded. Hamilton replied that £140,000 for eight public engagements in three months was not a bad rate of remuneration. Hamilton did not only highlight the cost, pro rata, of a Princess who was not much in demand for ceremonial occasions. He also attacked at the most vulnerable point: the relationship between public funding and private behaviour. As a person well rewarded out of public funds, he declared, the Princess was expected to have higher standards than other citizens. 'That has always been part of the case of pro-monarchists'.

Thus, there began - and it was to be a long road - the shift away from the justification for the Monarchy on 'model family' grounds. For Hamilton's view was no longer regarded as eccentric: on the contrary, it had become part of a new orthodoxy, echoed by the most sober voices. 'For better or worse,' as another Labour MP put it, 'the Monarchy is justified as a symbol of the social and family life of the nation.' If it abdicated this symbolic role, it would suffer, 'and its political role will change'. But if the Royal Family was no longer to be regarded as 'model', what did it offer? The traditionalist Peregrine Worsthorne suggested - and this was later to become a fall-back position - that it should be seen as 'normal'. Acknowledging 'a sad lack of private decorum' on the part of the Queen's sister, Worsthorne argued that royalty was bound to have occasional black sheep, and from time to time there would be 'royal broken marriages, merry widows, disorderly divorcees, delinquent teenagers'. The closer to the people the Royal Family became, the more affected it would be by the changing standards of a permissive society. The point was made in semi-jest. However, it was an argument upon which, with the passage of time, it became increasingly necessary to rely.

The Sun, enlivened by the 'working people' versus 'social gadfly' angle, claimed that the Queen had told her sister to choose, Duke of Windsor-style, between her royal duties and her relationship. 'Give Up Roddy Or Quit' it summarized. Undoubtedly, the Queen was concerned. 'She talked to me about her sister during the bad patch,' recalls an ax-courtier. 'She asked "How can we get her out of the gutter?" She was worried about her.' 'The Queen sometimes found it hard to cope with Princess Margaret,' says one of the Princess's friends. 'She wondered "What am I going to do with her?"' There were a number of meetings involving No. 10. At one point, a Whitehall paper was composed about the options, including the suggestion that Princess Margaret should be taken off the Civil List and allowed to lead a private life - in effect, the ultimatum issued to her over Townsend in the 1950s. 'The Queen was very resistant to this,' recalls a close Callaghan aide, 'and Jim was very supportive of the Queen.'

At the beginning of May 1978, Margaret was taken ill, and diagnosed as suffering from gastroenteritis and alcoholic hepatitis. On May 10th, it was announced that she was seeking a divorce. It was another rubicon. '. . . [I]f she divorced', predicted Philip Howard, before the announcement, 'her example would upset the established principles of Christian family life of the Church of which her sister is head, and attract the disapproval of influential Church leaders.' This was a fear. 'Mindful of the teachings of the Church...' Margaret had written in her devout little essay of renunciation, composed by her divorced would-be fiance, in 1955. However, times had changed, and the Church of England on this occasion treated her with gentleness. Dr Coggan, the Archbishop of Canterbury, expressed his sympathy.

The Roddy tragi-comedy took some of the edge off public reaction to the divorce, especially when it became clear that there was no prospect of Llewellyn becoming a member of the Royal Family. 'I am saying categorically,' he declared, in answer to the obvious question, 'that I will never marry Princess Margaret.' That was a relief to practically everybody. However, the blushes continued. In October 1978, Llewellyn launched a pop record, 'Roddy,' at the night-club Tramp, in the presence of twenty photographers. Even The Times printed a picture of the event, whose newsworthiness - as the debutant singer modestly admitted - had more to do with the company he kept than with the quality of his music. The record did not sell many copies. In December, Lord Snowdon re-married. In 1980, Princess Margaret's relationship with Llewellyn ended when, he, too, married somebody else.