Imagine the scene. Former bachelor buddies Paul McKenna and Simon Cowell sunning themselves on a Barbados beach, permanent partners in tow, Simon playing happily with his young son Eric, McKenna cooing in the background over the apparent domestic bliss.

Still tanned from his holiday and recalling the scene now, the irony isn't lost on multimillionaire McKenna, the 51-year-old newly-engaged self-help guru and hypnotist, who used to think relationships weren't for him.

"I said to Simon, 'Two years ago, who would have thought we'd be sitting here and you'd be playing with your son Eric and your two dogs?' I'm so happy. The pace of life is much happier for him now. We sit and talk for hours about different things. We don't talk shop, even though we do have some business dealings. He's been a very good friend to me over the years."

Meeting McKenna at his mews house in desirable Kensington, it's clear his recent engagement to Kate Davey, 43, his personal assistant of 20 years, has given him even more of a spring in his step, as has his latest self-help book The 3 Things That Will Change Your Destiny Today!, introducing 'Havening Techniques' which include lateral eye movement and rubbing your hands up and down your arms, in a sort of maternal hug.

They're designed to unblock trauma and stress from the past and promote positive thinking. They certainly seem to have helped clear his own lasting blockage, namely commitment phobia.

"I've always really liked Kate, but she's my assistant. Of course I fancied her, but she would be in a relationship or I would be in a relationship and it just didn't seem appropriate."

Indeed, McKenna often seemed to go for glamorous, high-profile women - previous girlfriends included TV presenter Penny Smith and model Liz Fuller.

"A life coach friend of mine said to me, 'I notice you date all these beautiful girls but you don't really like them and they don't really like you'. He then said, 'Why don't you think, who do I love to be with and who am I attracted to?'

"Well, I sat back and did a sort of Excel spreadsheet and it went, 'Boing - it's Kate!' I'm some pervy boss!"

Kate's former fiance had died suddenly, around the same time that McKenna's father Bill died a few years ago, and neither of them were in a good place, he recalls.

"Then one night, Kate and I had a couple of glasses of red wine and I said to her, 'Tell me something about you that I don't know', and she said, 'I love you'.

"I said, 'I love you too', but she said we couldn't do anything about it because we work together and it wouldn't seem right'."

So very properly, he took her on a date - and things progressed from there; 18 months on, they're engaged.

"At first, it felt very naughty and clandestine. She didn't want to tell people. Some people have been very happy for me. I've just had a text from Kirsty Young saying, 'Brilliant! You're engaged!'

"Kate's beautiful. She understands me very well and I find that liberating. I was on an endless pleasure quest when it came to relationships, but now I've found something that's fantastic."

He took a little time off in October to take her on a romantic weekend along California's Pacific Coast Highway, recalling how he went down on one knee, popped the question and presented her with a 'paste' ring (he'll have the real one made up for her).

The wedding will take place this year in Buckinghamshire. They haven't set a date yet, but it's bound to be awash with celebrities.

Davey, a pretty, down-to-earth blonde, who is not at all the glamour puss I'd imagined, shies away from the media spotlight but says it upsets them when critics make extremely personal attacks on her husband-to-be.

She tells me McKenna's really intelligent and that, at the end of the day, all he wants to do is help people. I'm sure he's more than happy to live a millionaire's lifestyle too.

They live together at his luxurious house in Hollywood, although she's kept her apartment, where she returns to when she needs some space.

"We still socialise with friends, but staying in is the new going out for us. We love watching Game Of Thrones and Peaky Blinders," he says.

"I honestly thought I would never find love. I just thought it wasn't for me. I've been in relationships that have lasted a few months and was in one which lasted a few years, but I wasn't very happy."

Cowell, who McKenna has known for 20 years, will be organising the stag do. "We'll just have a curry and go go-karting," the hypnotist reveals. "He's like an older brother."

McKenna wants to start his own family, although Kate is not so sure - she takes the blood-thinning drug warfarin, which she'd have to come off were she to become pregnant. "Kate has health challenges, but there are other ways - surrogates," McKenna suggests.

The man who has reportedly sold more books than any other non-fiction author in the UK, including I Can Make You Happy/Sleep/Thin/Rich), says the latest techniques he writes about helped him through clinical depression, particularly when his father died.

"I also lost a number of friends and was working with people who were depressed, and I got infected by their mindset," he reflects.

"I went through periods where I thought, 'If I have a bigger house or more money, then I'll be happy'. But that's pleasure, not happiness." I want to tell him I'd settle for pleasure.

"Happiness is the backdrop to our life," he continues. "Much as I love a consumers' society, which is what we live in, it's set up to continually show you what you don't have."

And yet it's undeniable that McKenna, who started his career as an in-store DJ at Topshop and came to prominence doing stage hypnosis mostly for comic effect, uses commercial opportunity at every avenue.

In collaboration with Virgin Holidays, he's recently been doing 'Holiday Hypnosis' at a major shopping centre, hypnotising people into feeling like they've been on a two-week relaxing holiday. He's also introduced his Hypnotic Gastric Band app, which could deliver the same impact as £5,000 of weight loss surgery, and has a plethora of other money-making projects on the cards.

Los Angeles suits him better than Britain, because of the Americans' sense of optimism and their belief that you can do anything, he says.

"I drive a Bentley in LA, and if someone pulls up at the lights in a beaten-up old car, the driver might say, 'Hey man, neat car. One day I'm gonna have one of those'. You'd never get that here. In England, they'd just key the car.

"In America, they have much more hope. The whole culture is one of self-improvement and betterment. They celebrate success much more than we do here."

He has a talk show in the US, interviewing people in what he calls a "psychological deconstruction". British guests have already included his pals Simon Cowell, Kirsty Young and Sir Roger Moore. Many other projects are in the pipeline.

For now, though, he is buzzing with thoughts of getting hitched.

"Being engaged has changed me. It feels different. We're committed," McKenna says. "When we're married I can't wait to say 'My wife...' It's so grown up."

:: The 3 Things That Will Change Your Destiny Today! by Paul McKenna is published by Bantam, priced £12.99. Available now