A few weeks later she lunched with some of her friends, including Cecil Beaton and Alastair Forbes. When she got up to leave she said: “I’d better be pushing along,” to which the ever-mischievous Forbes volunteered: “Haven’t you pushed along far enough, dear?”

The first five years of the Edens’ marriage were dominated by politics. Until they entered Downing Street, they lived in the Foreign Secretary’s official London residence in Carlton Gardens. She accompanied Eden to Paris and Washington and attended the State Opening of Parliament and party conferences.

Not long into the marriage, in 1953, Eden’s health was permanently damaged by a botched gall bladder operation which caused him lasting problems. Then in 1954 Clarissa suffered a miscarriage, as a result of which there were no children of the marriage. She spent several months with Eden at the Geneva Conference (on the crisis in Indo-China), after which Eden was appointed a Knight of the Garter, and early in 1955 she accompanied him to Bangkok for the SEATO conference.

The whole period was overshadowed by the agonising wait for Winston Churchill to surrender the reins of office and stand down as Prime Minister. Churchill found endless excuses to stay on. By the time he did so in April 1955, it was the 10th time he had promised to go. Right to the end he played a cat-and-mouse game about the date of his departure.

Arguably, Eden had waited in the wings as crown prince for too long. But soon after taking over he went to the country and secured the personal triumph of increasing the government’s majority from 17 to 60, the first incumbent administration to increase its majority for a century.

At Number 10, Lady Eden improved standards of catering for official receptions by bringing in new caterers, though her plans to redecorate the premises to the style of the 1730s were never realised. At Chequers, which she never liked, she took a keen interest in the grounds, ordering old-fashioned roses and planting fruit trees.

An incident when she politely asked the occupant of a farm worker’s cottage on the Chequers estate to move some washing, which was blocking a lime walk some way from their cottage, was interpreted in the press as an example of high-handedness.

The Eden administration had to put up with hostility from various quarters in the media. Harold Macmillan, no favourite of Clarissa’s, attributed The Daily Telegraph’s dislike of the Prime Minister to a “blood row” between Lady Eden and Lady Pamela Berry (wife of the newspaper’s then owner, Michael Berry), who, it was said, felt slighted by the Edens’ avoidance of her social functions.

Just before Christmas 1953, Lady Pamela had told the American columnist, J O Alsop, that she thought that Eden’s health was poor and precluded him from being prime minister. The remark was repeated to Duff Cooper (Lord Norwich) who duly passed it on to Clarissa. This caused some froideur between the two ladies, and led to what Clarissa’s cousin, Randolph Churchill, described as “a trail of unhappiness and misunderstanding”.

There is no doubt that Lady Pamela, the power behind the throne at the Telegraph, took it upon herself to destroy Eden, asking Hugh Gaitskell: “Is it possible to get rid of a Prime Minister in peacetime?” In her diary, Lady Jebb (later Lady Gladwyn), wife of the British Ambassador to Paris, made dark references to “Lady Macbeth” and alluded to “Clarissa’s war”. In private correspondence just after Suez, the Oxford historian Hugh Trevor-Roper declared that the “vain and foolish” Eden was “wholly managed” by his wife.

Highlights of Eden’s premiership included the visit of Bulganin and Khrushchev in April 1956. With her customary wry take on life Clarissa observed these and other heads of state.

Soon afterwards, the Suez crisis erupted, during which, as she wrote: “I felt I could only help by bolstering up without trying to lessen his load or demand that he rested.” She will be remembered for her remark in a rare political speech at Gateshead on November 20 1956: “In the past few weeks I have really felt as if the Suez Canal was flowing through my drawing room.”

This came to be considered symbolic of the Edens’ attitude to the crisis. In later life she conceded that “drawing room” was perhaps unfortunate.