The Mitford Girls: The Biography of an Extraordinary Family by Mary S. Lovell (Little, Brown, £20).

NANCY, Pamela, Diana, Unity, Jessica and Deborah.

They were the "mad, mad Mitford girls" and over three decades in the 1920s, 30s and 40s they both delighted and shocked English society.

All of them were honorables and half of them rebels. All six were undisputed beauties.

Nancy was the satirical novelist who based In Pursuit Of Love and Love In A Cold Climate on her own family and during the Second World War aided the authorities in imprisoning her sister Diana.

Pamela was the quiet English rose who was never happier than being in the countryside, had John Betjemen at her feet (he was the first person, in print, to describe the sisters as The Mitford Girls) and of whom Lord Mountbatten once said, she had ordered her Aga to match her amazing blue eyes.

Diana was the iconic beauty who made a prudent marriage to a Guinness heir (Evelyn Waugh based Vile Bodies on their set), then threw it away to embrace fascism and become the second wife of Blackshirt leader Sir Oswald Mosley.

Unity was the one captivated by Nazism, flirted with Hitler, pronounced herself anti-Semitic and put a gun to her head the day Great Britain and Germany went to war.

Jessica was the fanatical Communist who eloped to Spain with bad boy cousin Esmond Romilly (who by his own admission was "probably" Winston Churchill's son).

Deborah was the one who swore she would become a duchess and she did too - she's still the highly-successful Duchess of Devonshire.

The Mitford gels were both a product of their priviliged birth and of a country finding its way through the alluring concepts of social upheaval.

Their parents, the 2nd Baron Redesdale and his wife Sydney - who for a short period, with their first four children, lived at Batsford House, near Broadway - were not the daft country-house "Muv" and "Farve" that Nancy would have us believe in her novels... bellowing uncle Matthew and twittering aunt Sadie.

They may have failed to send the girls to school (but despatched only son Tom, to Eton); continued the family obsession of adopting absurd nicknames Bobo (Unity), Decca (Jessica) Woman (Pamela) and variously for Diana (Dana, Dina, Bodley, Nardy, Cord and Hanks) but they were mortified by the bizarre headlines that followed in the wake of their daughters' excesses.

The Mitford roller-coaster has been a fairground attraction for 80 years and Mary Lovell has not come off the rails in being seduced by its luminaries. She set out to explore the richness of their personalities, not judge them and in that, she succeeds admirably.

Here is a well-heeled family doing what the rest of us do: argue, fight, offend, hate, love and above all, have fun. She makes them seem quite ordinary.

But of course, ordinary is not what The Mitford Girls were.

Diana - now aged 91 and living in Paris - was always regarded as a bte noir and for a time, as she followed Oswald Mosley to prison without trial, was arguably the most reviled woman in England.

Unity, fatefully conceived in the Canadian gold mining town of Swastika, could perhaps have done nothing to prevent herself from falling under the spell of Adolph Hitler. Her suicide attempt in Munich's Englischer Garten on Sunday, September 3, 1939, failed, but her recovery was only partial and she died nine years later at the age of 34.

As Lovell points out, it was Jessica, who brought the family the most pain. Her long periods without communication, her wild outbursts to the Press, her refusal to see her father, her protracted feud with Diana - reconciled only during Nancy's final illness - was the cause of a deep and bitter divide.

History has lumped The Mitford Girls together, like some six-headed beast. They were sisters, brought up in the same way yet developing in such an individual manner and they took the 20th Century by the throat.

Probably, we shall not see their like again.

David Chapman