A suave waistcoated gent opens the door of the white stucco house in one of South Kensington’s smarter streets. I proffer my hand but he simply coughs: “Lady de Rothschild will receive you upstairs, sir.” Where were you brung up, Gerard? You don’t shake hands with butlers.

Manners matter here. I am to meet the American plutocrat-ess who has become Britain’s most influential political hostess. Climbing past modern masters I am confronted by a statuesque grand dame. Ah, she looks more the part: “How do you do, your ladyship.”

“I’m Victoria, Lady de Rothschild’s personal assistant,” she announces, more cut glass than Dartington. “Lady de Rothschild shall be down shortly.” Can I crawl under that priceless persian rug in shame, I wonder.

“Would sir care for chocolate brownies with his tea?” Jeeves inquires solicitously. “Baked freshly this morning.” Indeed sir would. Mmmm. Maybe this “I’m one of the wealthiest people on the planet” lark isn’t so bad.

Luckily Lady R is detained on yet a higher floor — poor deary, must be bushed managing all these staff — affording me snooping scope in the study. Head for the photos. Here she is with her great friends Bill and Hill (she threw a party for the Clintons in London recently) and there’s a sweet one greeting her (casually dressed) new best friend Tony, the prime minister. Poor Bono and Henry Kissinger make do with the cheap frames at the back. Oh, look, her neatly typed itinerary: Commons to see Tony Baldry MP, lunch at Harry’s Bar, view the forthcoming autumn collection of . . .

“Oh, Lady de Rothschild, didn’t see you there, charmed.” This time there is no mistake: huge smile, outrageously expensive blonde hair and fabulous long legs winningly packaged in a short Yves St Laurent skirt: the classic American beauty. And for 48, she looks disgustingly youthful (maybe money can’t buy love, but it can buy time). Still, she pays a high price: she doesn’t touch a yummy brownie.

Even in her own right Lynn Forester, as was, is not short of the odd few hundred mill. She made her money setting up her own company which secured licences for broadband radio in the US in the 1980s. (Later she became an adviser to President Clinton and helped to fund his third way project.) In 2000 she married Sir Evelyn de Rothschild, 71 (the Clintons let them spend their wedding night in the White House). Hubbie bumps along as the chairman of a merchant bank. Why do the rich always marry the rich? “I think,” she says huskily, “Evelyn wanted a partner rather than a parasite. Part of our happiness is we are both strong people who know we love each other.”

So you have to be part of the club to be trusted. Scary. But don’t dare call Lady R right wing: she says she would like to become a Labour fundraiser (she is already one of the Democrats’ most successful ever money-milkers).

So with those leftie views does she feel guilty about being so wealthy? “No, it’s not about being a limousine liberal. I work hard not only for myself, but for others, and I would fight to the death to preserve our way of life.”

Much later, before going to bed, she e-mails: “Wealth is not the enemy. Greed is.” Hmm. I thought you needed one to get the other. She is devoid of that British cringe about money: it’s fine to have servants, you are providing employment. Her youngest son, 15, attends an “elite school” in New York where his best friend is a Caribbean truck driver’s son on a scholarship “and he is going to make it”.

In Britain she has been impressed that immigrants become peers. Americans, even Democrats, believe in self-creation. It is the dream to come from nothing and make it big.

She has lived the dream. When she divorced her first husband in 1993 she hung onto the millions she had made and added more. A friend had challenged her: “How do you ever expect to fall in love between Gate 18 and Gate 9? You will be the world’s richest woman and nobody will love you.”

But she refused to change. In 1998 she flew to a conference in Scotland with her bag carrier Kissinger (he had a seat on her board). He introduced her to the head of the banking dynasty, “the miracle of my life”, who was smitten.