‘I am holding on to you,’ the princess whispered in reply. Her husband-to-be, Antony Armstrong-Jones, couldn’t have been more different from Peter Townsend, as he was a photographer. But at that moment at least he appeared to make her happy, and the Queen was pleased her sister had found what she thought was a more suitable kind of love. She remained the Queen’s adored sister and was very much a royal princess, never letting anyone forget it if they did not treat her with due respect.

During the difficult years leading up to the separation and eventual divorce of the Prince and Princess of Wales, it was Margaret’s turn to support her sister. By that time, she could feel pleased by the professional success of her own children, Viscount Linley and Lady Sarah Chatto. Linley had set up as a furniture maker, and in 1993 would marry Serena Stanhope, while Sarah became a painter and married a year later than her brother. For the first time in years, Margaret felt that perhaps her life had been a fulfilling one.

As a young woman, she always felt that the Queen was so good and perfect while she was very much the opposite, doomed to an unhappy marriage and a succession of unsuitable romances. Then she found a new kind of confidence and, for the first time in her life, she felt able to give the Queen advice, rather than the other way around. The ironic poignancy of this reversal of their situations was not lost on the Queen or Prince Philip. This was not how anyone could ever have seen things turning out, with Margaret at ease with herself and her children happily settled, while the Queen and Philip were faced with a parent’s failure in their own family.

Margaret had no sympathy for the Princess of Wales, despite the similarity of their predicaments. Once supportive of the princess, she became almost as vocal as Prince Philip in her condemnation. She felt Diana had let the Queen down. And if there was a touch of hypocrisy in her reaction – and Margaret’s life was hardly a shining example – there was no doubting the sincerity of her devotion to her sister. But if that was a comfort, it was not a solution. There was no escaping the fact that the early 1990s were joyless times for the Queen. At a time of life when she should have been enjoying the fruits of a lifetime’s dutiful labour, she was facing the gravest crisis of her reign.