It is not often you get a former ruler of the Kremlin partying on down at one of Britain’s most aristocratic stately homes. But that was what happened last weekend when Mikhail Gorbachev, the former Soviet president, was co-host at the most extraordinary social event of the summer: a gala night staged by a one-time communist spy at Althorp, the ancestral home of arch-aristo Earl Spencer.

One week on, the full story is only just emerging of how Alexander Lebedev, the mysterious Russian billionaire and former KGB agent, spent more than £1m throwing a private party with Gorbachev at the Northamptonshire estate where Diana, Princess of Wales grew up and is now buried.

“It was amazing,” said Geordie Greig, editor of Tatler magazine, who helped to organise the “Russian midsummer fantasy” last Saturday. “It was fabulous,” gushed another of those who attended. It was, said another, “almost surreal”.

The evening began with guests — including Orlando Bloom, Elle Macpherson, Salman Rushdie and Tamara Mellon — wending their way through the parkland of the 8,500-acre estate. People in 18th-century dress sat in the trees “like a sort of scene from a Watteau painting”, said one guest.

Strange figures with wolves straining on leashes roamed the grounds. A camel played nomad on the lawns. As champagne and oysters were served in front of the house, a troop of Cossacks charged from the woods and whirled gracefully into an “equestrian ballet”.

The Christ Church cathedral school boys’ choir sang from a balcony. As dinner approached, children appeared dressed as fairies; they played out a mime that drew the 400 guests into a giant marquee.

Jellied borscht with smoked sturgeon and “golden Oscietra caviar” were followed by free-range guinea fowl, all set off by a fine riesling and Crozes-Hermitage.

The musical entertainment was equally striking. Andrei Gavrilov, the international pianist and former winner of the Tchaikovsky prize, was followed by the hip-hop band the Black Eyed Peas. Bono, the U2 singer, chipped in by video link from Dublin.

The purpose of this extravaganza was — apart from general fun and games — to launch the Raisa Gorbachev Foundation, a charity dedicated to helping children suffering from cancer, named after Gorbachev’s wife who died from leukaemia.

Another of the guests was Robert Harris, author of Fatherland and Archangel. “It was quite a party,” he said. “Even the tables were revolving. Gorbachev didn’t dance but he seemed to enjoy the attentions of many people.”

After dinner the serious business of raising money began. Up for auction were lots including a personal tour of the Hermitage in St Petersburg, with the museum director as guide; a piece of diamond-encrusted jewellery donated by Christina Aguilera; a work of art by Tracey Emin, who was among the guests; a flight in a MiG-25 Foxbat, the fastest jet fighter in the world; and dinner with Gorbachev at Lebedev’s dacha outside Moscow.

By the end of the evening the event had raised £1m for the charity — and the party, organised by planners Urban Caprice, had cost even more. The charity, however, would get all the money raised because Lebedev was picking up the tab for the entire bash.

It left many guests and outside observers bemused and intrigued by their unassuming, bespectacled 46-year-old Russian host. Who is that guy, they asked? An oligarch on the make or a new philanthropist? And what, while spending £1.3m to raise £1m, is he really up to?

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