IN A small room in Somerset House, in London, Sam McKnight runs his hand over his bare head. “Luckily,” he says, “I never had a thing about losing my hair.” To be honest, I had wondered. For the last 40 years McKnight has teased, primped and pimped the hair of the world’s most beautiful people (models, supermodels, actors, princesses; you’ll know all their names). You might think it would make him a little self-conscious about the lack of any of his own.

It seems not. “I had amazing hair. I had hair down to here.” As he says this he is indicating the lower half of his body. “I had a feather cut. I had perms. Oh my god. Loved it. Amazing. I’m really glad I enjoyed it.”

But as is sometimes the way of things when you get older and you’re male, his has mostly gone. “It went slowly and I was fine. Thank God. Thank God! Because I’ve seen people who have an issue with it and they do plugs and things …”

He doesn’t say it but the hairstylist in him doesn’t sound convinced about the idea. “They do it really well now,” he protests, “but I’m glad I didn’t have to go through all that.”

He gets his phone out to show me his last hurrah back in the 1980s when he bleached his hair blond. “I just thought, ‘You know what? I’m not going to have this much longer. I’m just going to go out with a bang.'

“I had it for six months and I thought I was great in my Comme des Garcons black shorts, black polo neck and polka dot socks wandering the clubs of New York thinking I was Andy Warhol.”

He shows me the picture. More David Hockney perhaps, I say. “Yeah, one of those. Some would say Bet Lynch,” he laughs.

As far as I know McKnight never styled Bet Lynch’s up do (nor Julie Goodyear’s for that matter). But in his (almost) 40 years in the business he has worked with everyone from Kate Moss and Naomi Campbell to Lady Gaga to Diana, Princess of Wales, and, behind the camera, from Irving Penn to Nick Knight.

He’s just finished a Harper's Bazaar cover (the US version) with Karl Lagerfeld and Gigi Hadid. In short, even though he’s a miner’s son from Ayrshire, he’s slightly more Chloe than Corrie, more Erdem than EastEnders (we’ll come back to Brookside later).

If we accept that fashion is a team game, with the model and the photographer up front and the designer in goals, McKnight is more of a deep-lying central midfielder. His work knits the play – or in his case the model and the outfit – together. His is the art of the artful pin, the concealed weave, the flamboyant wig. He has crimped and combed his way around catwalks and fashion shoots, helping create signature looks (he’s the man who cut the then-Princess Diana’s hair short) and high-end imagery for Vogue and V and W and any other fashion magazine worth its Dolce & Gabbana ad spend.

But, to slightly labour the football analogy, he’s not fashion’s Joey Barton, shouting about what he’s up to. He’s more the silent Lionel Messi type. “I don’t have a huge ego. My work is to help create the best image. It’s not really about me.”

Well, it’s not been until now. Because the reason I’m here is a new book and a retrospective exhibition in the gilded confines of Somerset House. Here is McKnight pinned up, you might say. What, you wonder, does this all represent for him? “Well, 40 years of hard bloody graft. That’s what it represents.”

He makes no special claims for his work. Others do though. In the book’s introduction Karl Lagerfeld extols McKnight’s sense of humour and original eye.

“There are few artists (hairdresser is the worst word I can imagine),” Lagerfeld writes, “as gifted as Sam in the worlds of hair and fashion … His work has a special sensual expression.”

Both the book and the exhibition arose out of what online commentators would call an old media problem. “The book came about because I had all my old work in papers and magazines. I carted it around every flat I ever lived in.”

He decided to get it all archived and found an archivist at Somerset House who, on seeing it, suggested to the gallery that there might be an exhibition in it. That coincided with the offer of a publishing contract from Rizzoli.

And so here we are on the verge of a retrospective stretching back to 1977 which takes in supermodels and pop stars, actors and princesses, and none of them with a hair out of place. Unless that was exactly what was required. “It was a bit of a strange experience doing it,” he admits, “a bit like your life flashing before you. It was quite emotional.

“We had to write to every single person in the book. We had to track people down and I got amazing emails from people, models now in their fifties and sixties I hadn’t seen for years and are now … I don’t know, living in Idaho with three kids.”

As a result, the book and exhibition are a kind of pop history of the last 40 years of fashion, from Christie Brinkley to Kim Kardashian. For him, though, it’s more a reliquary of memories and friendships. He doesn’t seem to have a bad word for anybody. “It’s all about collaborations and relationships. We’re stuck together in the same space touching each other for a long time. It’s probably as close a relationship as you have in your life apart from your partner or your child.”

His own childhood was spent in the 1960s in Ayrshire. Let’s go back, I say. Let’s discuss the glamour of New Cumnock. “Oh God. I don’t know if those words go together,” he says.

“Glamour to me as a kid was Moira Anderson on The White Heather Club. And she was our music teacher at school, funnily enough. I remember her wearing a fake leopard coat. That was glamour to me. Or a bus conductress. My auntie was a bus conductress and she had a big beehive which was very, very glamorous.”

McKnight was 15 when he saw David Bowie drape his arm around Mick Ronson on Top of the Pops in 1972. The impact of that vision is still embedded in his psyche. “We’d suddenly gone from dirty old hippies to this amazing Technicolour spectacle of futurism which we hadn’t really seen before.”

Somewhere in it he saw his future; something more exotic than the possibilities offered by the teacher training college he was attending. “I hated it. I absolutely hated it. I felt like I was still at school. My friends owned a hairdressing salon in Prestwick and to get a bit of money I would help them on Saturdays and during the holidays. I very quickly discarded college and then in a few months moved to London.”

How did giving up college go down with your parents? “Oh God, not well. Not well for about 15 years, I think. It took them a long time before they thought I had a proper job.” Things have changed. His 84-year-old mum was down the week before we meet. “I’m hoping she can manage the opening.”

For the younger McKnight London had a pull to it. He had a friend from Muirkirk who worked for Biba and the idea of the big city appealed to him. “London was pretty depressed then too but there were these little pockets.”

He worked in a few salons but reading Vogue he noticed that the covers were all styled by employees of the salon Molton Brown. He deliberately sought and got a job there and soon was doing his own styling for the magazine.

“And then in 1980 I kind of thought, ‘I don’t want to work in a salon at all.’”

A year and a half later he was in New York styling for the American fashion magazines. “I got there in 1982 for the first time. It was March and everything was freezing and it was brown and everyone was still wearing flares and platform shoes. It felt like you were in an episode of Hill Street Blues from 10 years before. It was still stuck and it was that way until the end of the 1980s.

“It felt like it was in a time warp until things started moving downtown and then the whole business grew and exploded. I probably got there at the right time.”

He spent the 1990s working with the supermodels and being the personal hairdresser to the Princess Diana after being introduced to her by the fashion photographer Patrick Demarchelier.

He tells me about the first time he saw her. “This long-legged blonde comes up the stairs in a studio in Hackney and bounded in and immediately charmed us all because she was a proper presence. But really lovely and funny. We had a great day laughing.”

She spent that day sitting in a white dress and wearing a tiara and after the shoot, at her request, he cut her hair off. (Years later he’d do something similar for the model Agyness Deyn.)

That meeting with Di would lead to a long friendship. He would style her hair most days. “I’d do her hair in the morning if she had something to do and she’d sometimes have a function on a Saturday night and I’d go and do her hair.

“I spent the next seven years on and off with her and I saw her grow up into this lovely, confident woman from almost a little girl.”

Is it true they watched Brookside together? “She did love Brookside. I have very fond memories of her doing her hair and watching Brookside.”

Can he remember how he heard about her death? “Yeah. I was at home. I got a phone call from Naomi [Campbell] and Donatella Versace in the middle of the night asking me what had happened. They were on a photoshoot in Milan.”

It’s a sad memory, but it is also very Planet Fashion, isn’t it? But then that has been McKnight’s world for so long.

In the 1990s he was on hand for the rise of the supermodel. He worked with them all. What’s his definition? “I think it’s overused. There really aren’t many. Those girls who are truly super have an ethereal beauty about them. They might not look it if you see them in the flesh. They’re not the most conventionally pretty girls. In fact, they’re definitely not. They’re more creatures.

“They’ve got long limbs, long necks. They were probably the ugly ducklings at school. But it all works on camera. The camera loves them. They love the camera. It’s an art. Like a silent movie star."

They have a special appeal, he suggests, “some kind of unknown magic that people are mystified by. And a slight vulnerability about them at the same time. I think all of them have some kind of vulnerability. I think when it’s too aloof it doesn’t work."

You say that, but aren’t some of them – Naomi Campbell springs to mind – divas too? “Yeah, there’s always that side of them. But Naomi’s lovely. She’s a lovely, lovely thing. And loyal and kind. People are many different things. The public like to be told they’re this character. It’s normal.”

Where is McKnight in all this? He lives alone, spends time in his garden when he can. He’d like to spend more. But mostly he is working. It’s a Friday when we meet. When is he next on a plane? “Monday.” Has work been life for you, Sam? “It has. That’s fine.”

He doesn’t feel he’s missing out. “You develop really good relationships with people.” He starts to talk about “Kate”. I’m presuming he means Kate Moss, not the Kate who works in Moss Bros.

“Kate, I’ve known since she was 17. A lot of these girls come to me as teenagers. You see them growing up and you become the father figure. And then the grandfather figure. It’s a really fulfilling role.”

You might consider that that’s the best kind of hair extension.

Hair by Sam McKnight is published by Rizzoli, priced £35. The exhibition of the same name opens at Somerset House on November 2 and runs until March 12 next year.