For the duration of the Serpentine Gallery’s summer party, a line of guests, strawberry martinis in their hands, queue outside a black Portakabin. Geri Halliwell, in born-again-virgin white, snuggles up to her new squeeze, Evgeny Lebedev – the scion of Russian oligarchs, with the dirty allure of an indie rocker. Rod Stewart has the complacent smirk of a man living out his sexual fantasies, as his new wife, Penny Lancaster, lap dances to the accompaniment of the Seventies disco music. There are posh models Jasmine Guinness and Saffron Aldridge; a Hollywood contingent of Thandie Newton, Kim Cattrall, Pierce Brosnan and Diane Kruger, (Kevin Spacey arrives late, post-theatre), artists Damien Hirst, Tracey Emin, Dinos Chapman; fashion designers Giles Deacon and Phoebe Philo; and an interchangeable crew of fine-boned girls with heels sinking into the turf and boyfriends in sunglasses, defiant of the dusk.

I know it’s not the Portaloo queue: I’ve just seen David Walliams in that, disaffectedly kissing women who squeal up to greet him. And on the top step of this particular oblong hut is the small, agitated figure of Tatler editor Geordie Greig, charming yet insistent, ushering in some, turning away others – the world’s poshest bouncer. They are lining up, I learn, to be photographed by Dennis Hopper, most renowned as the oxygen-masked monster in Blue Velvet, but here tonight as a major collector of modern art, the host of the party and its semi-official photographer.

Every time I ask someone to define high society, there is much talk of “party heat”. It could be said that this makeshift booth is the white-hot point in an evening that’s the thermo-nuculear centre of the summer season in London, which everyone seems to agree is, right now, the hottest city on the planet.

While other red-carpet events – those endless magazine awards ceremonies and movie premieres – secretly pay the glamorous to show up (or at any rate fly them over from LA first-class and put them up in Claridge’s), celebrities attending tonight have paid to come. And even then will only have their £250 accepted if approved by the Serpentine party committee.

But on the gathering’s outer fringes are those who are here not because they love art so much as need it. As Tatler editor-at-large Vassi Chamberlain says: “The Serpentine is where the rich come to buy cool.” Proximity to sweary, drunken, iconoclastic artists spatter them with bohemian edge, just as attending fashion parties sprinkles them with vicarious chic.

Outside the Ladies, an upper-class beauty stands forlornly sponging martini off her borrowed couture. Brow-lifted, fat-backed matrons shiver in the evening chill. A hedge-funder’s drunken girlfriend loudly bemoans her schedule: “He’s flying in from St Petersburg but only for 24 hours. It’s crazy! I could kill him. And I still haven’t sorted out our dates for Ibiza.”

High society is, at least superficially, more open than ever before. It is no longer about being well-bred, attending the right school or having an Hon. “That gave you a certain entrée 40 years ago,” says Geordie Greig. “It was a much narrower pool. Now you can be anyone. But there is no guarantee of getting in – it’s not down to whether you have a certain amount of money or houses, but comes down to a mixture of things. It’s about having an energy.”

Connections help, certainly. No wonder the upper classes have flocked to the social networking site Facebook, which allows you to see who a person knows, but not – without their permission – entry into their circle: a virtualisation of real high society.

These days, a dull but gorgeous girl won’t get on Tatler’s 100 Most Invited list. Neither will a rich bore, the cheesily famous or the merely titled. Because, to have what Greig describes as “social oxygen”, a person must tick at least two of the following boxes: beauty, breeding, fame, talent, money or “edge”.

Pop celebrity or humble roots are not necessarily an impediment. Trinny Woodall is a prime-time star, but is proper posh with mighty connections, as demonstrated by the six-figure sums she blagged from richer friends on Comic Relief does the Apprentice. Before she became a weather girl, Tania Bryer grew up in Knightsbridge, a privileged Jewish princess. Russian model Natalia Vodianova was a fruitseller from the Gorky slums, but has graced Vogue covers and is married to Justin Portman, a real-estate heir. Politicians, it must be noted – even the well-connected and upper-class, such as David Cameron and George Osborne – steer clear of the party circuit: there are too many tabloid bear traps, it undermines their egalitarian intent and these days they work too damn hard.

But of all high-society entry criteria, the greatest is money. The Joseph Rowntree Foundation revealed recently that the divide between rich and poor in Britain is at its widest for 40 years. But that statistic gets nowhere near describing the champagne fountain of cash that is currently cascading through London. Recent market wobbles aside, the non-doms (non-domiciled rich who live in Britain tax-free), the expatriate Russians and the hedge-fund managers who have lately acquired fortunes of dynastic magnitude have transformed the upper echelons of high society in which they aspire to mix.

This wealth has given birth to fundraiser galas inspired by the glittering US model of red-carpet philanthropy, raising amounts for charity unprecedented in Britain. The Ark Foundation Ball, at Marlborough House, organised by Swiss banker Arpad “Arki” Busson, produced £28 million for underprivileged children from guests who paid £5,000 a table to be entertained by Madonna and Prince and listen to Bill Clinton give the key-note address. At the Raisa Gorbachev Ball at the Lebedev residence this summer, J.K. Rowling, Tom Wolfe, Anish Kapoor, Ralph Fiennes, Naomi Campbell, Elle Macpherson and Tom Stoppard mingled in a cocktail of old and new money, high and low culture.

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