Paul McKenna has spent the past decade promising people that he can make them thin, rich, confident, smarter, quit smoking and sleep in a series of best-selling self-help books. He has even fitted some willing readers with a "hypnotic gastric band".
His 17th book, Instant Influence and Charisma, published this month, contains a system to unlock the "glowing human energy" that is charisma. "When you apply it, you will make more money. You will get dates more easily. You will win more arguments and make more sales. Above all, you will be more free to express your own values and live a more personally rewarding life."
Sounds good. So, first question: is Paul McKenna charismatic?
"I'm told I am," says McKenna, with no hint of a pause. "I think of myself as charismatic, yes. I think I've learned to be, because as a child I didn't feel that charismatic."
Do many children go around feeling charismatic? Never mind, there is definitely something about McKenna the grown-up.
At 52, he has made a fortune of £65m - "that's apparently what I'm worth but I'd have to ask my accountant!" - mainly from his books and self-help seminars. He is also a celebrity favourite, having helped David Walliams to swim the channel, Sophie Dahl to lose weight and Ellen DeGeneres to stop smoking. There are many more examples, and they come up often.
"Roger Daltrey uses my confidence CD before he walks on stage. Not because he needs any confidence, he's fantastically confident. And Russell Brand uses it before he performs sometimes."
The weird thing is, the famous hypnotist with the super-slick patter, big gold watch and designer suits, the man who inspired Little Britain's Kenny 'look into my eyes' Craig, is a bag of nerves face-to-face.
He doesn't sit still for a second - scratching his head, taking his trademark black glasses on and off, itching his nose, jiggling his knee, sometimes stuttering. When I listen back to the interview later, though, that famous voice is as calm and measured as it is on a hypnosis tape. Was all the fiddling one of his distraction techniques? I don't know, but I didn't dislike him. He reminded me of Alan Partridge.
We meet in his London house in a Kensington mews. The living room is like a creamy hotel suite with black velour sofas and a few photographs - of his dogs, his late father, McKenna with Al Pacino, McKenna with Sting and so on - scattered around. There is a handful of engagement cards on the mantelpiece, though he got engaged to his PA, Kate Davey, more than a year ago.
We retire to the "red room", an archetypal therapist's lair of crimson sofas and walnut furniture. There's a McKenna crest on the mantelpiece and a giant gold crucifix and a Ganesh figure in the fireplace. Is he religious?
"I'm not religious, but I do believe in God. I do believe in something greater than myself, I don't think this is all an accident. I pray every day." He looks thoughtful. "Mostly I give thanks rather than ask for something."
This isn't his proper home - that's in Los Angeles.
"I found this fantastic house which has a lot of Hollywood history," he pauses.
Go on, who lived there?
"Well, the Rolling Stones made a movie there that was never released. Who else? Sean Connery. Richard Gere and Cindy Crawford. John Schlesinger owned it and used to rent it out to people. When I first moved there I used to have a barbecue every Sunday and invite all the Brits around ... I was a single man, I had a good life."
That's all over now. He wants to move back to Britain.
He revealed last year that he chose his fiancée by spreadsheet. Davey had been his personal assistant for more than 20 years, but it was only when he entered her vital statistics into Excel that he realised, "good God, I love Kate!"
It wasn't just a spreadsheet, he demurs, his mother also said he should date her.
"When I rang her and said, 'I've got something to tell you, me and Kate are in a relationship', she went, 'goody, goody, goody!'" he laughs, absolutely delighted.
He proposed in Big Sur in October 2014. He hired a Maserati and drove her along the Pacific coast highway before getting down on one knee after dinner, with a £1m ring.
"And she said yes!" He slaps his thighs and kicks up his feet like a child. They will marry in Buckinghamshire this year in a friend's house.
Will it be a big do? "Yeah, and it will be a fairly eclectic mix of people from different worlds - from the arts, science and sports."
McKenna has quite a lot of famous friends and he has no qualms talking about them. Indeed, he interviewed most of them on his American talkshow on Hulu in 2014. His LA gang includes "Paul Oakenfold, DJ and producer, Sir Ken Robinson, the most fantastic mind on education of our time and I'm good friends with Simon Cowell and Ryan Seacrest, who is a remarkably talented broadcaster."
Cowell, in particular has been a "great counsel" to him and will be his best man. "I often ask Simon for advice. He's a very clear thinker."
Has he ever been starstruck?
"Once, David Bowie. I was so blown away that I couldn't ... The president of any country could sit in front of me right now and I wouldn't let it affect me. But I got a bit starstruck with David Bowie."
He tells me that he doesn't charge for his one-to-one sessions, instead asking patients to give a donation to a charity. His real money is made in global seminars - a one-day weight loss course costs around £300 - and books.
The new book - he writes at least one a year - is the product of 20 years studying the psychology of influence. Broadly, it's a guide to how people get what they want by using certain postures, verbal tics, mirroring and rapport.
McKenna's upbringing in Enfield, north London was happy. His mother was a teacher - "charismatic, very intelligent, very positive" - his father a building contractor - "a very dignified man, very kind". He has a brother who is a building contractor and lives in Enfield. He was a nerdy child, a bit insecure but "pretty popular". He hated his school, St Ignatius in Enfield.
"It was very violent. It was a place where I saw cruelty and I think it gave me a taste for compassion. When I left there I had a lot of Catholic guilt so I wouldn't feel comfortable about enjoying things.
"It wasn't a happy time in my life. I think the Jesuits are very good mind-manipulators - I learnt a little bit about it from them."
After school he got a job on Radio Topshop, moving on to Radio Caroline and Chiltern Radio. He also did voiceovers.
It was interviewing a hypnotist on Capital Radio that changed his life. He borrowed a book and started hypnotising people in pubs for a laugh.
It led to a glittering television career in the 1990s, but in time Cowell advised him that he would have to leave entertainment if he wanted to be taken seriously.
The controversy of Christopher Gates, who sued McKenna in 1998, alleging that his participation in a live show had triggered schizophrenia, may also have played a part. McKenna won the case, but estimates that it cost him around £4m in lost earnings.
Does he ever do the silly stuff - making friends fall in love with a broom etc - just for fun at dinner parties?
"No, I haven't done it for many years. I see that as the me of the past."
That said, last month Cowell bid £3,000 at a charity auction in Barbados for a McKenna hypnosis session for his wife, Lauren Silverman, joking that it was to cure her of her "shopping habit".
Has McKenna helped hypnotism to be taken more seriously?
"Definitely. It's now part of the A-Level psychology syllabus, I believe, since I've been on television. Anywhere we have done studies, it's usually about 7 in 10 people that my techniques work for, which is way above average. I think I can cure most psychological problems most of the time."
He has picked up a few professional rivalries along the way and the first chapter of the new book has a dig at Allen Carr .
"No I don't rate Allen Carr," he confirms. "Well, we never got along really. Allen was not very friendly to me."
Then there's his spat with Derren Brown, who said something rude about McKenna in an interview and then emailed him asking: "Are we still friends?" McKenna emailed back: "Well, you're the f**king mind reader."
The last year has been trying. In August, his bid to have a lurid lawsuit thrown out of court in America failed. His ex-fiancée/manager, Claire Staples, is suing him for millions, claiming that McKenna was addicted to cocaine, alcohol and prescription pills. McKenna denies the claims and is counter-suing in the High Court.
On January 2 this year he allegedly made an air hostess cry when he was refused alcohol on a British Airways flight, for which he has apologised.
Has he had therapy? "Of course."
One last question then: if I could write a book for Paul McKenna, what might he need help with?
"For me? Do you know, if anything I'm a little bit tough on myself at times. That just came into my head just then! I think maybe I could be kinder to myself."