You may feel like you know David Gandy better than you know yourself. Unless you’ve been avoiding ad breaks, billboards, bus stops and all printed media, you will have seen him, arms stretched, biceps bulging, supine in nothing but a pair of crisp, white and relatively tiny M&S undies. The 34-year-old from Billericay is everywhere.

One of the world’s few genuine male supermodels, worth an estimated £10m, Gandy is the person who almost single-handedly put muscles back on the catwalk.

But his work is not just restricted to the upper echelons of haute couture. With his Autograph underwear collection, he’s been tasked with putting the spark back into Marks & Sparks. Whether even he, and a silk knicker-clad Rosie Huntington-Whitely, can transform the retailer’s fortunes is yet to be proved, but Gandy call it the perfect gig.

“M&S has been fantastic to work with,” he says. “It once had this old-school image but everything is changing; it’s nice to be a part of that. There’s a lovely buzz about M&S right now. It’s an exciting place to be!”

But with Marks’ mumsy image, did he have his reservations? “I’ve always wanted to work with M&S but the timing never seemed particularly right,” he admits. “I really wanted to work with a British brand, which is actually quite hard because there aren’t that many high-street brands that are properly British.”

I was never the successful one with the girls, there were always guys who were more successful than me, at school and at university. I thought they were better looking than me. They’re not models, but they’re great looking guys

The brand, says Gandy, is a perfect fit in more way than one. He may be a multi-millionaire and date supremely gorgeous women – he has recently rekindled his relationship with The Saturdays’ Mollie King – but Gandy is a salt-of-the-earth kinda guy. You won’t catch him in designer kecks. “I’ve been asked by many brands to design underwear, but I thought it would be great to do it with M&S. One in four men wear M&S underwear; they’re the underwear experts."

Gandy put his crash course in underwear design to good use. “I had the final say, and it’s the first time that I’ve put my name to anything,” he explains. “I’m known for high-end – Dolce, Savile Row – so I wanted my product to have a luxury feel, without the price tag. I had full control, from the quality to the stitching to the branding, which I wanted to be very subtly but classily done. And I wanted it to be understated, classic underwear, that doesn’t say Beckham or Calvin Klein. I wanted to get away from that and tap into the buyers who wouldn’t go for something so flashy. Or cost £45, which I don’t agree with paying. I want you to pay around £20, for the best underwear, in the best packaging.”

He even reveals he turned down design houses to work with Marks & Spencer. “I believe in affordability,” he tell us. “People assume my wardrobe is just Dolce and the likes. I don’t spend that much on clothes, or as much as people would think."

But does Mr High Street have his limits? What about Primark?

“Ehhh… no, nothing from Primark, I probably work to that cut-off point. That’s very disposable, I don’t really believe in disposability,” he smiles.

He may prefer his high-street brands, but it’s no surprise onlookers expect Gandy to be dressed in the very best – after all, he made his name, in earnest, from a Dolce & Gabbana advert. But it’s also not the first time he’s been oblivious to his own suave sensibilities.

“I was never the successful one with the girls, there were always guys who were more successful than me, at school and at university. I thought they were better looking than me. They’re not models, but they’re great looking guys,” he says, modestly.

When Gandy’s friend entered him in a This Morning model search – which began his career as a model – he was flummoxed. “When people at college heard I was doing it, they told me, ‘You’re known as Model Dave on campus.’ First I’d ever heard of it!” he laughs.

He says he was, for a while, “quite gangly” – see the Internet for evidence of his pre-supermodel look – before he played a “tonne of sports” and filled out. It seems odd to imagine it now, but his muscle-bound look was probably considered a hindrance back then.

Androgyny was all the rage when he first hit the fashion scene. And then there was Gandy, a towering hulk of a man. Most would have slimmed down, but Gandy says he was encouraged to do the opposite: what’s the point of going with the grain?" he says.

Despite the risks, that’s exactly when his big break came: with the Dolce & Gabbana Light Blue campaign.

The advert saw him climbing elegantly out of the blue Mediterranean onto a yacht moored off the coast of Capri, clad only in a briefest of white swimming briefs. He became an immediate star, for obvious reasons.

Eight years on and Gandy has continued this highly lucrative association with Dolce, while fronting additional campaigns for Lucky Jeans, Banana Republic, Jaguar and Savile Row’s Henry Poole & Co. He also became the first male to ever be nominated for the British Fashion Council’s Model of the Year.

I don’t like being centre of attention. I hide how nervous I am. When I turn up to a red carpet, I still get butterflies. At awards, nine times out of ten, I want to say to the driver, ‘Just keep going, I want to go home.’

He is now at the stage where he’s turning down Hollywood acting roles. “I was offered Fifty Shades of Grey, 300 2, Hercules, which a lot of people probably would have taken as actors but I’m in the fortunate position where I like what I’m doing. I don’t feel the need to act,” he explains. “I know if you’re in a successful film, your status changes overnight. And I’m not saying I wouldn’t, I’ve read many parts that I’d love to play, things that I’ve chosen, though, not things that’ve been sent.” But surely he was tempted by Fifty Shades? “Critics and literary experts aren’t fans but I respect it for what it is. But was it right for me? No. It just didn’t feel right, in any way. And in my gut, it felt right to say no,” he says with a diplomacy that’s as well-crafted as his six-pack. “Sometimes it’s about what you say no to, as opposed to what you say yes to that can lead to success.”

One ‘acting’ job he did take was in Jennifer Lopez’s ‘First Love’ video. Raunchy isn’t quite the word. Despite the beautifully shot and effortlessly cool outcome – he and Lopez embracing in grainy black and white – Gandy remembers something a little more awkward.

“It wasn’t the greatest!” he admits. “Very difficult filming conditions. High winds, cars were being moved on set, sand pelting you. The crew were gaffer-taping their glasses to their face while we’re just rolling around in T-shirt and a dress. It was freezing cold by the end of the night. But Jennifer was great; we had a laugh doing it. You just roll around and laugh – and shiver at the same time!”

Did the inimitable Gandy get even a little starstruck around the music icon?

“No,” he says. “I’ve met her a few times before and she’s perfectly nice. She’s a person, just like everyone else.” Gandy seems rarely fazed – or at least, he’s good at hiding it if he does: “I don’t like being centre of attention,” he says. “I hide how nervous I am. When I turn up to a red carpet, I still get butterflies. At the GQ Awards, I got nervous; lots of butterflies. Nine times out of ten, I want to say to the driver, ‘Just keep going, I want to go home.’ Some people thrive on it, but not me.”

Gandy admits to having insecurities still, which seems rather implausible. “Everyone does; I’d be very suspicious of someone who says, ‘I’m perfect.’ So yeah, I’m always trying to improve on my body. You’re not going to be in as good shape as you were eight years ago, it’s harder these days.

“When we shot for the Marks campaign, I was training for a good six weeks, and was very proud of the result but you know, it’s hard. But don’t get me wrong, I enjoy doing it; it’s all a part of a process.”

Sometimes it's what you say no to, as opposed to what you say yes to, that can lead to success.

What’s notable about Gandy is that despite him laying himself bare on billboards up and down the country, he’s remarkably private otherwise. He bats away any questions about his love life. “Look, I’m a private person, naturally. I don’t tweet pictures of my dinner or let people know of my whereabouts. I’m just naturally private,” he says.

But is he happy? “I’m always, always adapting,” he says. “I’m never fully satisfied. You know what, sometimes, I wish I would just enjoy that moment, and think, that’s great.

But I’m always thinking, ‘Could I do another collaboration, could I work with another charity,’ moving on and on and on. That’s what I thrive on personally. I’m in a business where I can do so many different things. I’m lucky.”

Recently, Gandy was voted the second best male model of all time by Vogue – behind American Tyson Beckford. “I was gutted about that one,” he jests. “But c’mon, there’s no shame coming in second to Tyson. He really is one of the nicest guys in the industry.”

So what’s next for David Gandy? As keen to work behind the camera as he is in front of it, he has recently moved into retail ownership. He was such a fan of his David Preston boots that last month he bought the company. The company had its soft launch at January’s London Collections: Men.

We have no doubt this next project will be a great success, too – boots’n’all.

Read the full interview in this month's issue of Square Mile here.

Marks & Spencer’s David Gandy for Autograph collection is available now.