Tomorrow there will be a memorial celebration for Jessica (`Decca') Mitford, writer, wit, aristocrat, socialist and famous sister. John Mortimer, who is writing the screenplay of her autobiography `Hons and Rebels', recalls the woman he knew
It was last year, in San Francisco, that I first listened to the tape, made by the aptly named "Don't Quit Your Day Job Records Ltd" of Jessica Mitford and Maya Angelou singing that unforgettable duet "One Fish Ball".

First came Jessica (always known as Decca) Mitford's unmistakably upper- class mezzo singing:

"The waiter bawled

Right down the hall

We don't give bread

With one fish ball ..."

And then came the controlled scream of Maya Angelou slagging off the trembling diner and again telling him not to expect bread.

Two elderly women of huge talent from different corners of the earth, the daughter of an eccentric English peer and the naval dietician's child from St Louis, Missouri, were enjoying themselves together. And there was one thing about Decca you could be sure of, at any time, she would be doing something unexpected but entirely characteristic.

When we met for dinner, I asked her about the earthquake which had shaken her house in Oaklands, California. "I was peeling potatoes when the tremors started," she said, "and I rang a friend and asked her what she was doing, and she said, `Peeling potatoes'." So Decca came to the entirely rational conclusion that the act of peeling potatoes brings on earthquakes and resolved to avoid doing so in the future.

Jessica Lucy Mitford was born in September 1917, the sixth child of Lord Redesdale, who was in the habit of calling any young man who visited his daughters a "sewer rat" who "stank to merry hell", who would seize any doctors he found about the house by the throat and "shake them like a rat", and who considered any criticism of hereditary peers an attack on Christianity. "After all," he said, "I inherited my title from my father just as Christ inherited his from God." Lady Redesdale also mistrusted doctors, taking the view that "the good body" could cure itself. Both parents agreed that sending girls to school was a waste of money, so governesses were engaged and paid from the sale of eggs at the large, modern and dauntingly ugly stately home Lord Redesdale built at Swinbrook. One of these governesses, not much good at academic subjects, would take the girls on shoplifting expeditions to Oxford. When Decca wrote her brilliant autobiography Hons and Rebels, it's important to remember that the word "Hons" didn't refer to her status as a peer's daughter but was the children's name (the Mitfords were great ones for nicknames and private languages) for the prolific hens who paid for their education.

From this household, daughters emerged who were rarely out of the headlines. Diana, the great beauty, married Oswald Mosley, leader of the British Fascists, and spent part of the war in prison. Unity, whose second name was, appropriately, Valkyrie, having become the close and admired friend of Hitler, shot herself in the head on the day that war was declared, but lingered on leading a strange half life until after Germany was defeated. Nancy became the friend of Evelyn Waugh and an entertaining novelist. Decca was always smilingly, unself-consciously Decca, and the youngest, Debo, who was reputed to have sung "Some Day My Duke Will Come" during her childhood, married the Duke of Devonshire. Only Pam seems to have lived a quiet life, and Tom, the one boy, was killed fighting in Malaya. But, for the most part, the girls seemed to have emerged from the eccentricities of Swinbrook feeling that the restless, war-torn world of the 1930s and 1940s was undoubtedly their oyster.

What I found most touching, in writing a film version of Hons and Rebels, was the extraordinary love two sisters, Decca and Unity, who flew to opposite ends of the political spectrum, felt for each other. Their room at Swinbrook had hammers and sickles at one end and photographs of Hitler at the other; they fought, sometimes physically, but remained each other's favourite sister. When Unity, horrified that England and Germany would be enemies, shot herself, Decca wrote of her as "my big, bright adversary".

I remember growing up in the 1930s and getting Out of Bounds, the magazine circulated by young Esmond Romilly, Churchill's nephew, to convert us to socialism, contempt for public schools and hatred of the Officers' Training Corps. Deeply impressed by all this, I became, for a while, a one-boy communist cell at Harrow. Decca fell in love with the idea of Esmond Romilly before she met him, and soon after she did they ran away to the Spanish Civil War together. They remained, for the rest of his short life, wonderfully happy together; and it was a sign of the power of the British upper classes, and the magic of the Mitford girls, that Anthony Eden sent a naval destroyer to rescue Decca from the clutches of Esmond and the Spanish Republicans. The captain tried to tempt her on board by offering her a chicken lunch, but the hungry Decca remained resolute and the destroyer had to return to England with no Mitford on board. In a parallel gesture, when Unity shot herself, Hitler arranged for her to return to England by a special train through a Europe at war.

It was a war Esmond regarded with as much mistrust as he had the Corps at his public school, and he and Decca retreated to a bar in Miami, where he mixed doubtful cocktails and she was a waitress. But, when he decided that Unity's ghastly friends had to be defeated, he joined the Canadian air force and was killed in action. He was 22 years old. Decca married the American labour lawyer Bob Treuhaft, with whom she lived happily until the day last year when she rang her sister and, her cancer having been diagnosed, said something like, "It's a bit of a bugger, but I've only got a month left to live." She was brave and funny to the end.

In the last chapter of Hons and Rebels, Decca wrote: "Too much security as children, coupled with too much discipline imposed on us from above by force, or the threat of force, had developed in us a high degree of wickedness, a sort of extension of childhood naughtiness. We not only egged each other on to ever-greater baiting and acts of outrage against the class we had left, but delighted in matching our wits with the world generally; in fact, it was our way of life." This is typically self-deprecating and no doubt contains a great deal of truth. But there's no doubt that Decca had a deep belief in social justice and human rights, as well as finding most things, even death, a possible source of laughter. That's why her story is so refreshing in these grey years when no one believes much in anything any more A memorial celebration for Jessica `Decca' Mitford will be held tomorrow, 5.30pm, at the Lyric Theatre Shaftesbury Avenue, London W1. Tickets available on the door on a first come, first served basis