The magic words, "I’m here for L’Oreal", secure me entry into an enormous, elegant private house in London’s Portman Square. It is bigger than the Shelbourne. Amazingly, someone lives here.

In the basement is an infinity pool. In the hallway, a clock softly tick-tocks. In the room I am led into, there are white carpets. I expect to see Dr No stroking a white cat. On the sofa are some Union Jack-patterned cushions. Over the matt black marble fireplace is the first indication of who I have travelled to London to see: on the 50in plasma screen there is a reel of Cheryl Cole extolling the considerable virtues of L’Oreal Paris. Tall women in heels and white jackets are going hither and yon, politely offering the most delicate sandwiches and cakes in the history of food.

It was far, far from this that Cheryl was reared: she grew up in a housing estate in Newcastle, the daughter of a painter and decorator.

I, with some other journalists, am soon led upstairs to an even bigger room, which houses a piano and a giant table, to meet Cheryl. She is not yet in the room. The girl from Elle magazine is anxious, because she has to see her editor for a meeting straight after this. We sit around the table and await the arrival of the star. On it is a tray with a selection of delicate cakes. No one dares eat a single one.

Suddenly, Cheryl Cole appears in the doorway, like an old-style movie star. Think Vivienne Leigh, or Rita Hayworth. Or Audrey Hepburn with a remarkable Northern English accent — and that hair.

Well, she has just become the new global ambassador of L'Oreal Paris. James Joyce devoted four pages of A Portrait of the Artist as a Young Man to interpreting Thomas Aquinas's theory of beauty. Had Joyce been in London on this otherwise ordinary Tuesday morning, he would only have had to gaze upon Cheryl Cole’s bewitching pulchritude to understand completely beauty in full, chic bloom.

There isn’t a hair out of place when the tanned renaissance woman, who graced the cover of British Vogue in February, enters the room for our beauty tete-a-tete.

I am meeting Cheryl to talk about her role as the new star of the Elvive Full Restore 5 range. The Geordie goddess is the first Brit since Kate Moss, in 1998, to sign on the dotted line for L’Oreal Paris. I have frantically rehearsed my questions on the plane over, even going so far as to practise the beauty questions on the Aer Lingus air hostesses.

Cheryl takes one look at my unruly titian tresses and quips that I desperately need the L’Oreal hair products. “You could use some Elvive,” she says. I feel like drowning myself in the infinity pool.

Everybody but me has an immaculately posh English accent, except for the person who has brought us all here, Cheryl — who still, endearingly, talks with a common-as-muck Newcastle brogue. I have also met her before, which helps. I spent the day with her and her co-workers in Girls Aloud in Dublin in January 2004.

Luckily, she remembers. How could she not? We had lunch in the Herbert Park Hotel in Dublin, after I met them at Samantha Mumba’s 21st birthday party at the Four Seasons the previous evening. They were only kids, really. Yet apart from the brash, opinionated hurricane that their presence was in the restaurant, there was no escaping that a huge part of their immense and understandable appeal was that Cheryl et al were young and lovely.

“We’re five young girls,” Cheryl smiled back then, “Why not be sexy?” Why not, indeed? “Young kids who haven’t been brought up in a nice home, or who maybe are from a council estate, look at us — we give them inspiration,” Cheryl said in her refreshingly thick Geordie accent. “‘If you can do it, maybe I can do it.’”

Like a modern take on Sylvia Plath’s The Bell Jar, Cheryl sang on No Good Advice: “Daddy told me look into the future/Sit at your computer/Be a good girl/But I dig the music that I’m making/I flick my finger to the world below”. The song’s expression of a young woman’s dream of getting something more out of life than a secretarial course seems to chime with the ambition of the working-class hero and all-round Tyneside wonder woman Cheryl has become today in London. Look at her now. Top of the world. Her style is copied the world over by the fashion-conscious universe. All this and L’Oreal too.

“I keep saying the word ‘unreal’ because that’s how it feels. I grew up watching L’Oreal. That phrase, ‘Because you’re worth it’, is iconic,” she says today with no understatement.

“I feel really honoured to be a part of all this. I mean, you look at the other women who are doing the L’Oreal ads and it feels like a dream. ‘Should I really be doing this?’ you think. I feel absolutely honoured!”

She says she had to say, “Because you’re worth it”, a good few times in the ad before they got the perfect take. “The first couple of times I giggled,” she says. “I was fluttering. Because I had this thick Geordie accent. It felt really unreal that it was happening. So it did take a few takes!”

I ask her did she ever practise in front of the mirror doing the L’Oreal ad when she was a teenager growing up? “Saying, ‘Because you’re worth it’? Oh yeah,” she smiles. “That’s what I’m saying. It was one of those things that you always joke to your friends about. It is just one of those iconic phrases. I’m sure most girls would love to say those words. Oh, the iconic-ness of being a L’Oreal girl.”

And how does it feel? “It is very strange but amazing.”

Cheryl has famously big hair. Would she ever go for Posh Spice-style bob?

“There is a bit of a sore subject,” she says. “Because when we first got in the band — changing from the programme Pop Stars: The Rivals to becoming a pop band — everyone was like: ‘We are going to give you a makeover. What would you like?’ I was, like, ‘Go short. Cut it off,’ at the time. I was nearly two stone overweight and I cut me hair off. All I can envision now from me having short hair is a moon face. I identify having short hair with a fat face. And it is not a good look” — the girls from Elle and Now and Grazia nod in agreement — “and it is not versatile. Ask me after three children when I’m pulling it out. But yeah, I don’t think I’m brave enough.”

Luckily, then, Cheryl’s tresses are absolutely fabulous today. “My hair is my comfort blanket,” she says. “It makes you feel glamorous. It is the hair I dreamed of growing up. Where I was growing up, everyone wanted to grow their hair to their bums. That was the in thing.”

My own sister, Marina, wanted to be Farrah Fawcett as a teenager. Who did Cheryl want to be?

“I wanted to my friend from up the road,” Cheryl says. “She was called Julie and she was always really glamorous. Really glamorous, like she had the very best of new clothes on. She’s a mother now but she’s one of those yummy mummies. Yeah, I wanted to be her.”

What colour was her hair?

“Blonde! You see, you always want the opposite to what you are. She has two little ones now. I still admire her. I still think she is a very glamorous woman. She was positive and she was always smiling. As a kid, I looked up to her.”

She also says that she admires the eternal style of Audrey Hepburn. She is going to see Anna Friel as Holly Golightly in the West End production of Breakfast at Tiffany’s. “But overall, in terms of style icons,” Cheryl adds, “I just like people. I think there is nothing nicer in a woman than an air of confidence and self-belief.”

That kind of air that is innate, I suggest, rather than available in Bond Street shops.

“Yeah, exactly,” she smiles. “It is not that you put it on. It is something that oozes out.”

The girls from the glossies interrupt my increasingly intimate bonding session with Cheryl to ask about her beauty regime. I try unsuccessfully to contact my inner beauty queen, attempting to keep up. In any event, I learn that olive-skinned and dark-haired Cheryl never waxes her eyebrows. She only ever plucks, primarily because, she says, “every few days they are like tree trunks. I have a lot of hair.”

I also learn that she always uses a L’Oreal hair mask three times a week: “It smells fantastic and you can just slap it on.

“Because of what I do,” she adds, “I’m constantly having me hair blow-dried — you know, the heat and all that — and I’ve always taken care of me hair. It is a massive thing for me to take care of the condition of my hair.”

She says one of her key beauty activities at night is “getting the make-up off before I go to bed, even if I am shattered and lying in bed or I have been at a shoot all day where they have been reapplying make-up all day for pictures. I actually feel dirty lying in bed. You know that feeling where you wake up where you have mascara on your pillow?” Cheryl says — addressing the girls, as I am clearly baffled at this stage. “I can’t bear it. That’s one of the thing, even if I come in at 4am, I have to take off my make-up.”

Could you not employ someone to take your make-up off at 4am when you stumble in after a night? I ask.

“I wish I could! But no, even if I’m shattered, I will take my make-up off. You know, washing your face always wakes you up a bit, doesn’t it? But it has got to come off.”

My inner beauty queen also learns that Cheryl’s eyelashes, contrary to speculation, are real. That’s not all. In terms of foundation, she is a strong believer in less is more.

“It is like a security thing, but sometimes it looks not so nice when it is thickened on. That’s why I think I’m obsessed with L’Oreal True Match Roll’on,” she says, as I become more lost than those girls in Picnic at Hanging Rock.

“My dad is a painter and decorator. It reminds me of one of his roller-brushes. I was sitting in the car putting it on and I was like ‘Oh my God, it’s on!’ That is one of my latest obsessions.

“You obviously can’t use that,” she says to me. “But you could use the hair mask. That mask would do down very well with you.” Once she gets her famous visage clean at night, she continues, “then I thicken my face with moisturiser.”

“As I am getting older,” says Cheryl, who was born on June 30, 1983, as the girls look on, spellbound now, “I am realising that women use should less foundation if you can get away with it — as in, foundation that you can still see your skin through — is far nicer and more natural. It is just better.”

How do you switch off? I ask. Not that I want to switch her off, or anything, but I am a boy.

“Watching EastEnders and Coronation Street. I sit at home in my grey satin trousers — they’re not tracksuit bottoms, but they are the most unattractive yet most comfortable trousers ever — and a pair of fluffy slippers and a hoodie, with a nice cup of hot chocolate. That’s my favourite. I am slightly obsessed with hot chocolate at the moment. Me mother just got this stuff where you have to boil the milk, but the powder has got real chunks of chocolate in it.”

A glass of Guinness would relax you too, Cheryl, I say, like the stage Irishman trying to ingratiate himself with the posh English gels from the Elle and Now — all of whom are doubtless wondering who this Paddy freak is and what, precisely, he is doing here at their beauty pow-wow with Cheryl Cole. “

Guinness? Would it? Sheridan’s is my latest trick. It is basically like Baileys. It is coffee and cream liqueur. Oh, wow, that with a bit of ice.”

“How many would you have?” I ask. I am being slightly bigoted now, thinking that all Geordie lasses are binge drinkers.

“I’m not a big drinker,” she answers, “ I just have the one.” This is presumably a different Cheryl to the Cheryl who told me in 2004 in Dublin that she drank shots of slippery nipples. “After my third slippery nipple,” she said, “my memory wipes. It erases. So I try just to have a couple and then keep up with vodka and Red Bull to keep my alertness, You got to have some awareness.”

Cheryl’s headmaster in school allegedly once coldly turned to her and said: “It will be interesting to see what you do with your life, Miss Tweedy.”

The short answer is that Miss Tweedy — now even more famously Mrs Cole since her 2006 marriage to Chelsea footballer Ashley Cole — travels on the front section of planes a lot, and I don’t mean Ryanair — and has become an international superstar, both as a pouty chanteuse with Girls Aloud and as a judge on the ITV talent show, The X Factor.

And now, if that wasn’t dazzling enough, her CV also includes the new face of L’Oreal, for which the Geordie no-hoper was allegedly paid ¤800,000. And she's worth it.

- Barry Egan