'But the girls - I was expecting them to be almost naked'

"You must admit," says Tara ("a very, very good friend of Prince Charles", I am later informed), "you must admit it's all terribly daring!"

We look around the room: the Savoy Riverside Room, a courtly and refined reception area, leading to a lavish banquet room. There's the Marchioness of Milford Haven, and there's Sir Anthony and Lady Bamford. And they're all wearing pyjamas! It's a pyjama party! A society pyjama party! Pyjamas!

"It's a great leveller," says Tom, Tara's friend. "When everyone is in pyjamas, you don't know whether they're a Viscount or a servant. You don't know whether it's Old Money or No Money. You don't know who they are. Pyjamas are the great leveller."

While Tom's sentiments are well-intentioned - I'm sure - I'm not wholly convinced of their factual veracity. Firstly, you know exactly who the Viscount is - it's Viscount Linley, and he's surrounded by people being extremely nice to him. Secondly, you do know whether they're Viscounts or servants, because the servants haven't been invited. And thirdly, when everybody is wearing immaculate, flawless silk pyjamas and cashmere dressing- gowns emblazoned with the crests of Harrods and Hacketts, you've got a pretty good idea of where they stand in the Old / No Money stakes.

"You know," says Kate from Tatler, "Hacketts have completely sold out of pyjamas! People have probably spent more money on tonight's outfits than they did at Ascot."

"It's the most wonderful idea," says Kate's friend, Pam. "People have been saying for years that we must have a society pyjama party, but it's taken Kate's vision to pull it off."

The table plan reads like a Who's Who of the siblings of the great and good. There's an Oppenheimer here (Emily), a Sassoon (Adrian), a De Cadenet (Alain). There's even a Von Bulow in the room - Cosima von Bulow to be precise- who, judging by the family movie, Reversal of Fortune - comes from a lineage of people well used to wearing pyjamas for long periods of time.

"Good heavens above," says an unidentified marchioness, looking me up and down - clapping her hands in delight at my battered old T-shirt and shorts (foolishly, in retrospect, I've gone for authenticity: this really is what I wear in bed). "How funny," she says. "What a marvellous idea."

"Thank you," I say.

"It really is contrary," she says.

"Thank you," I say.

And so the evening progresses. Unfortunately, I am neither landed nor gentry enough to have been invited to the dinner itself, nor the apres- dessert disco, nor the fleet of bedrooms that the various young aristocracy have rented upstairs "in case we get lucky", but I am none the less absolutely welcome to "drink a little champagne and eat as many gherkins as I like" until 9.00pm.

"You want to know why I'm wearing a baseball cap with my pyjamas?" says Tom.

"Yes," I reply. "Why?"

"It's because I'm a real goer," he says. Then he pauses. "No, that's wrong. I'll start again. You want to know why I'm wearing a baseball cap?"

"Why?"

"Because I'm a real goer in bed. That's funny, isn't it?"

"Yes," I say. "Are you going to use it on the girls?"

"I certainly am," he replies, "and I've got a room booked upstairs. But the girls - I was expecting them to be almost naked."

"Were you?"

"Yes," agrees Tom's friend, Simon. "When I went to a student pyjama thing - very low-key, non-society, kind of middle-class - all the girls got naked eventually."

"Did they?" says Tom, looking aghast. "Did they really?" He pauses for a very long time. "You know," he finally says with a hollow chuckle, "I suddenly feel a little less privileged."