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You know a fashion label has serious cachet when its catwalk show is scheduled to start before 10am and every one of the world's Devil-Wears glossy magazine editors is in her seat, on time, present and correct.

That was the case at the Marni catwalk during Milan's Fashion Week.

Neither the hour nor the fact that Marni doesn't advertise in fashion magazines - the quid pro quo arrangement that ensures bums on seats - can dissuade them.

Marni is the closest thing that status-obsessed Italian fashion has to an avant-garde brand. Its characteristic round-shouldered separates in quirky prints and organic-style details - such as chunky wooden beads the shape and texture of pebbles - are the antithesis of the designer uniform of the arriviste.

The curious part is that despite its insider status, Marni is effortlessly, gloriously easy to wear and shaping up to be one of the decade's success stories.

Not that this makes the publicity-shy designer Consuelo Castiglioni, 52, any more comfortable being the public face of the brand that she founded with her husband in 1994. Even its name deflects attention from herself - it is the nickname for Castiglioni's sister-in-law Marina.

"I prefer that you look at the clothes - and not me," Castiglioni says of the public duties of a fashion designer.

The Marni headquarters in Milan are, like every Italian fashion HQ, bland on the outside, a grey block on a quiet street, but Castiglioni, originally from Switzerland, says: "Everybody who's born in Milan, hates it, but I like it. You can be at the seaside in an hour."

Into one of its bright-white rooms, Castiglioni appears, pretty and slight, with long, tawny hair and dark eyes. She wears her own black silk faille dress, printed with an abstract design, over-the-knee ribbed black socks and platform sandals.

Inevitably, Marni draws comparisons with another, far larger, quirky Milanese brand. But where Prada's collections switch dizzyingly from orientalism one season to sportswear or bourgeois the next, Castiglioni is more constant.

"It's not our philosophy to be one thing in one season and then state the opposite the next - that's what people like about the Marni brand."

Castiglioni revels in seeing women wearing her past collections. "I like people to be able to keep their Marni clothes and match them season after season, wearing the new ones with old ones. If I changed completely, you wouldn't be able to do that."

Lucinda Chambers, the stylist and fashion director of British Vogue, who has collaborated with Castiglioni since the brand's launch, attributes the insider status to the fact it attracts women who aren't fashion-driven as such, but curious about clothes. "They have the confidence and ability to mix pieces in their own way."

Although Marni's appeal is more covert than its Milanese peers, Gianni Castiglioni, Consuelo's husband and company chief executive, wouldn't have it any other way.

"Our target is a niche market," he says, particularly in regard to the brand's lack of advertising. "I don't think our customers expect to see us in every magazine."

Marni is in many ways a typical Italian family business. Consuelo Castiglioni was born in Lugano, just 45 minutes from Milan, over the Swiss border, and as a child excelled at science, rather than arts. Her father worked in the metal industry.

It was the man she met aged 19 and married at 25 who had the fashion connections.

His family owned the Ciwifurs fur house, which makes designer furs for Dior, Prada and Roberto Cavalli. When Ciwifurs decided to launch an in-house label it was Consuelo - despite her lack of formal fashion training, but described by her husband as a born designer - who was given the creative lead.

Besides Consuelo and Gianni there are six family members in the company, including their daughter Carolina, 25, and son Giovanni, 19.

The first Marni collections were only fur - shaved, dyed and patchworked. Never mind that Consuelo was reportedly a vegan.

Gradually the designs reached beyond the original concept and Marni became more closely associated with vivid, painterly prints and an eclectic appearance.

By 1999, British fashion journalists were noting the emergence of Marni's Notting Hill-ites drawn to the label's then hippie-luxe ponyskin clogs and quilted jackets in exuberant prints. Although Marni then moved on from the faux-peasant charm of early collections, Castiglioni says she was frustrated that her designs were described as hippie-luxe.

British women, then Japanese and Americans, were among the first to embrace Marni. But the sex-and-status-obsessed Italian fashion industry was initially nonplussed by Castiglioni's loose-fitting sack dresses.

"In London maybe they have a better eye and are more used to seeing interesting combinations between materials, prints and shapes," Castiglioni says. "In Italy it's more traditional."

Castiglioni's approach to design is unorthodox and intuitive. "Rather than either sketching designs or draping fabrics directly on a mannequin I talk a lot to the pattern-cutter. Then she creates the pattern and works on the mannequin. Then, I try it on." If she doesn't like the way it feels, it isn't made. Therein lies the brand's quiet strength. Marni is essentially a very feminine label that feels soft on the body rather than constricting, that chooses original-looking prints rather than the leopard spots loved by Italians.

Sparkling crystals and corset detail have no place in the Marni lexicon, which has meant a low profile on the red carpet. Celebrities who wear Marni tend to be those with a highly developed sense of their own personal style - Maggie Gyllenhaal and supermodel Amber Valetta - rather than teen starlets. As niche as it may be, Castiglioni admits her brand can't be complacent about celebrity endorsement.

"Celebrities are more traditional in their choice of eveningwear. And sometimes I think that a woman who chooses Marni chooses it for herself and not to be looked at. It's not that we're not sexy, maybe just less revealing."

Central to Marni's upward trajectory is that the label's creator and business brains are completely in harmony. Well, almost. "The part where we really don't agree a lot is the shoes. Men don't like wedges and platforms."

"He's more classical about shoes. But I prefer to be daring."