His first novel, The Scarlatti Inheritance, was published in 1971. A Book of The Month selection, the paperback rights were sold 'for more money then I thought was in Saudi Arabia'. He produced a novel a year until 1980 when he slowed down to one every two years. The books, all with trademark three-word titles like The Osterman Weekend, The Rhinemann Exchange and The Icarus Agenda, have sold in their millions. Indeed, with publication in 28 countries and 32 languages, Ludlum has sold 195 million copies worldwide.
Hidden by a billowing net curtain, he's having a mild coughing fit over by the open window of his suite at the Connaught, giving you a chance to calculate what he's worth, even if the royalty on his 195 million sales were, say, as low as 10 cents a copy.
The portly man emerging from the net curtain in blue blazer, striped shirt and silk cravat, one hand hefting a cigarette in a nifty holder, the other extended in greeting, is a seriously wealthy man. 'I certainly never expected to make a living from writing,' he says, settled on the sofa and puffing his cigarette. 'I thought after a while I'd probably teach, with a little writing on the side as supplementary income.'
He attributes his success to good timing. 'When I started the only people who were doing what I was doing were Mr Fleming, on a surface level, and Eric Ambler. Everybody was writing self-indulgent angst-ridden stuff. I think people were sick and tired of writers pounding their chests and saying 'Oh what a terrible thing the world has done to me'. But I love storytelling. I'm a fan of Dickens, Thackeray and the Russians.'
Dickens and Thackeray never quite got round to the kind of non-stop action adventures Ludlum writes. 'The Ludlum trademarks,' according to an unflattering notice in Publishers' Weekly in America, are 'improbable bloodbaths, repetitive action, stilted and off-the-point conversations and - most annoying - the use of italicised words or entire paragraphs to simulate passion'. But you keep turning the pages.
His latest, The Scorpio Illusion (HarperCollins, pounds 14.99), about a beautiful but deadly terrorist's plan to topple the United States government, scarcely pauses for breath. As the convoluted plot goes on its giddy way, it seems as if his hero, Tyrell Hawthorne - out of time, on the edge and over the top, a man who never runs when he can 'race' - can trust absolutely nobody. There are bloodbaths, lots of exclamation marks and the aforementioned beautiful but deadly terrorist thinks, whispers or shouts Muerte a toda autoridad three or four times a chapter. And you keep turning the pages.
'I don't take myself seriously but I take my writing very seriously,' Ludlum says. 'I write my books the best way I can. I'm always competing with myself. My favourite book is always the one I'm working on. When it's finished I always think 'This is it, this is the one'. A couple of weeks later I start another book and I say, okay this is it.'
He laughs and lights up another cigarette. Ludlum, a celebrity for almost 25 years, has a fund of amusing, often self-deprecating anecdotes he has honed on the US talk-show circuit. He even throws in an impersonation of Michael Caine (star of the film version of Ludlum's The Holcroft Covenant) in Dick Van Dyke Cockney. He chainsmokes and coughs as he talks, and only occasionally sounds as if he's telling his stories by rote.
Because of his tough-guy heroes and his books' focus on intelligence agencies, he is often presented as a hard man with a background in or links to the CIA. Much is made of the fact that at the age of 14, as the actor son of well-to-do Republicans in New Jersey, he tried to enlist in the Navy Corps under a stage name to participate in the Second World War. He was found out, but at 17, he did enlist in the Marines, although the war ended soon after.
He is the first to debunk the image. 'I do have friends in the intelligence services. They read my books to see the kind of adventurous lives they should be leading but aren't] As for the Marines - oh gosh, when you're young you do foolhardy things.'
He wrote his first novel when he was in the Marines, aged 19. 'It was not so much a novel as an experience of a young man seeing the world. Maybe 300, 400 pages. I lost it. My friends and I arrived in San Francisco and had a drink or two. I tell you, we weren't drinking lemonade. I woke up on the ferry. My shoe, my war ribbons and my manuscript had all gone overboard.'
Ludlum shelved the writing and went into the theatre with his wife, Mary, an aspiring actress he married in 1951. Summer repertory, Broadway, off-Broadway, New York television. 'I was always either a homicidal maniac or a lawyer,' he recalls. 'I always thought there was a connection.' He gave up acting to concentrate on producing at theatres in New Jersey. 'I enjoyed acting enormously but you've got to have a gypsy soul to make a success of it. I never had that. Nor was I that good. I mean I never put Mr Olivier on relief or anything.'
In the late Sixties he began to do the voice-overs for top TV commercials until he had earned enough to take 18 months off to write a novel. 'I swapped a lifestyle where I used to go to bed at four in the morning with a bottle of whisky for one where I get up at 4.15am with a pot of coffee to write through until 11.30. Getting up so early is not so that I can write, as Time magazine stated, 'with the creative mists of dawn'. I get up then because the phone isn't ringing.'
He has made so much money he doesn't need to write at all. He and his wife can do as they please, but they choose to spend summers in a house in Connecticut and winters in a beach-front flat in Naples, Florida, so that Ludlum can work in comfort in his offices attached to both. 'If I didn't enjoy writing, frankly I wouldn't do it. But what else am I going to do? You can't fall into a vat of whisky. Well, I could, but I won't'
Ludlum's London stop-over is part of a European promotional tour. Two years ago he covered Australia and New Zealand, though an accident while dancing 'apache'-style to 'I Love Paris' with his wife on their 40th wedding anniversary meant he toured those countries on crutches. (His portliness is explained by the fact he has been unable to exercise for a year).
He enjoys the international celebrity. 'Sure I do. I was an actor after all,' he says. 'But my family don't let me take it seriously. In the Bahamas recently, people were coming up to me and asking me to sign their books. One of my sons kept saying 'Oh there goes dad again - he's always pretending to be that writer guy'. I have three children. Two are in music and this son is an entrepreneur. I'm a Democrat, and frankly I'd throw him out if he didn't have a sense of humour.'
Ludlum's own sense of humour didn't really transfer to the page with his two farces The Road To Gandolfo and The Road To Omaha, although he says that 'two fantastic stars want to do Omaha as a film'. In conversation, or in anecdotal mode, he is at his best telling stories against himself. 'I was a guest on a talk-show in the Midwest once where the host described the plot of his favourite Ludlum novel. I listened with interest as he explained at great length the entire plot of Frederick Forsyth's Day of the Jackal.'