By any standards Mary Soames was one of the most remarkable women of her era: close confidante (possibly the closest) to Winston Churchill throughout the second world war, dedicated political wife, one of the most outstanding British ambassadresses sent to Paris, successful (against all reckoning) chairman of the National Theatre, and — later in life — a prize-winning author. She was also one of only three non-royal Ladies of the Garter in recent British history, and a Spectator contributor to boot.

All this went hand in hand with a reluctance to talk about herself, and — except on rare occasions — about the war, and the father whom she adored and was especially close to. It was not until late middle age that she was persuaded by friends to write about her parentage. In 1979 she published her first book, Clementine Churchill: the Biography of a Marriage. It was an instant bestseller, winning the Wolfson history award. She followed up with five others; the last, A Daughter’s Tale, published only two years ago.

She also brought back into print her father’s brilliant short book Painting as a Pastime; in which I like to think I may have had some small influence. Mary was a friend of many years’ standing. But that friendship was always modulated with more than just a proper amount of respect, plus a touch of apprehension. She was capable of great affection, indeed emotion. But you did not trifle or take liberties with her; let alone offer any ill-judged criticism of the father, ‘Papa’, whose memory she venerated. As her son, Nicholas Soames, once remarked, ‘Mary could break a swan’s wing with one blow of her nose!’

She was capable of delivering a stern rebuke for what she considered any falling short in standards. For all our long friendship, I have to record once or twice falling prey to at least a twitch of her nostril. Once, on signing a book, Mary looked at my ill-formed scribble and snorted: ‘What’s that?’ I replied, feebly: ‘That’s my signature.’

‘That’s not a signature,’ snorted Lady Soames. ‘It’s disgusting.’

On another occasion I had been commissioned by a tabloid to write a eulogy of WSC in wartime. I was stumped by a well-known quotation; I knew I had the words right, but couldn’t label the precise context. Experts like even Martin Gilbert failed me; so I turned to consult Mary’s encyclopaedic memory. She again couldn’t help. Rashly I said: ‘Never mind, it’s only for a tabloid; I’ll fudge it somehow.’

There was a terrifying snort at the other end of the line: ‘Alistair, you’re not a journalist, you’re an eminent historian. Now go and get it right.’ I crept back to the drawing board, reflecting that Mary had not been a sergeant in the wartime womens’ army, the ATS, for nothing.

Once Mary came to stay at what was a sad time for her. To cheer her up, I took her to tea with a mutual friend nearby. On the way I was riveted by her account of life in the wartime ATS, how she had been much happier in the ranks as a sergeant than, later, as an officer. So I clean missed the turning, despite having been there umpteen times. ‘Alistair, don’t you know the way? Didn’t you do map-reading in the army?’

One evening with Mary that I remember most particularly was 5 June 1994, the 50th anniversary of the eve of D-Day — 20 years ago. Several of us were reminiscing as to what we remembered. Someone asked, ‘What were you doing, Mary?’

Suddenly a previously closed door was opened. She said: ‘I can remember exactly. I was with my ack-ack battery in the south of England, and had been taken to a “hop” by a fellow officer. Afterwards he walked me back five miles to my billet, kissed me chastely good night, and walked back five miles to his billet. Then, as dawn came up, I heard the bombers roar overhead. I knew this was it, and I just knelt down and prayed.’

‘So had you had known the date? And that it was Normandy?’

‘Yes, Papa would discuss everything over the dining room table at Chequers. We knew everything.’ (Mary then would have been only in her early twenties.) ‘But we didn’t talk. Nobody talked in those days. Occasionally if something was very, very top secret, my mother would say, “Now Mary, that’s labelled.” But nobody did talk.’

Apart from being keeper-of-the-secrets, no one knew as much about Winston Leonard Spencer-Churchill in the second world war as did Mary. Moreover, she performed an unquantifiable service — worth the equivalent of goodness knows how many battleships and divisions — in keeping Winston, with his known vulnerability to the ‘Black Dog’, on course. And in saving his much-tried generals from going bonkers. Even while away with her battery she was in almost daily touch with ‘Papa’, cheering him up with constant affection and encouragement. For that alone, the world owes her a great debt of gratitude.