It was their forty-eighth wedding anniversary and that night the Queen carried on as if nothing had happened. Accompanied by Prince Philip, she attended the Royal Variety Performance at the Dominion Theatre in London starring Cliff Richard, Des O’Connor and Elaine Paige. True to her character of forgiveness, the Queen still retained a smidgen of sympathy for her troubled daughter-in-law and, despite Prince Philip’s misgivings, informed Diana the invitation to spend Christmas with the royal family at Sandringham still held firm.

Diana was not sure what to do and kept changing her mind. When she finally telephoned the Queen to tell her that she would not, after all, be spending Christmas at Sandringham, it was the final straw. This was the moment that Philip took charge. According to one eyewitness, he went ‘ballistic’. Just the mention of Diana’s name was enough to send him into a tirade. Princess Margaret joined the chorus, and so did the Princess Royal.

In any other marriage, it would have been the husband whom one would expect to deal with a situation of this kind. But the royal family is an institution unique unto itself – and the Prince of Wales, like his mother, is not very good at dealing with difficult or unpleasant issues. By default, therefore, it was left to the Queen to face up to the matter and deal with it as best she could. There was no other course open to her, Philip pointed out most firmly. A divorce was the only solution to this untenable mess. And as sovereign, it was within the Queen’s authority to make that clear.

Philip almost had to guide her pen hand across the paper, but at last it was done, and on the morning of Monday 18 December, the letters were hand-delivered to the Prince and Princess of Wales demanding they seek a divorce. Diana was taken aback by the firmness of the Queen’s tone. But it was no longer a matter for her to decide. In giving the Panorama interview, Diana had taken the future out of her own hands. The Queen had reached breaking point and divorce, as painful in its details as its inevitability, was the only option left.