Sipping coffee in the boardroom of the spectacular circular space that makes up the Net-a-porter offices in west London, Massenet looks low-key but pulled together. She is wearing a delicate, breezy black dress with opaque tights and fantastically frivolous chunky Marni heels decorated with glass beads. 'I'm a big fan of quirky shoes. I find shoes entertaining because they're so temporary. You can indulge in a trend for a few months then move on to the next,' she says, glancing at her feet. 'And I'm loving the dress trend at the moment because it's so easy. You can just throw on some tights. I never want to think about what I wear.'

Massenet, 40, set up Net-a-porter in 2000. She was working as a fashion journalist and researching the new trend in online shopping when she became exasperated at the choices available. 'There was just nothing for a fashion-forward woman to buy,' she recalls. Massenet canvassed some designers and received a resolute 'No' to her suggestion that they make their clothes available to buy online. So she decided she'd do it herself and launched Net-a-porter from her kitchen table.

Three years ago sales topped GBP11 million (HK$170 million), and they've doubled every year since, with customers in more than 70 countries, from Denmark to Dubai, snapping up Burberry Prorsum dresses and Chloe wedges, and receiving their goods the next day. It's this gamble, founded on 'enthusiasm and blind faith', that won Natalie the British Fashion Award for Best Retailer in 2004.

What's her secret? 'I love fashion,' Massenet enthuses. 'And I think we at Net-a-porter have an emotional reaction to the products we see on the catwalks. That's what makes us pick a certain piece. It's a serious business based on emotion and passion and an adoration of fashion.'

Part of Net-a-porter's appeal - the laser-like selection of key items such as the Chloe Paddington - is inspired by Massenet's approach to her own wardrobe. 'If the consumer saw the amount of products out there, she would be overwhelmed,' she says. 'But because I'm very edited in my own wardrobe, I think I apply that to the site and the selections we make. The modern woman has her basic wardrobe, but she comes to us for something fresh each season.'

Massenet herself adheres to a strict daily uniform: jeans, jacket, white shirt, black shades and frivolous shoes. She updates the items in her wardrobe regularly to incorporate the changing tides of the fashion calendar, meaning the jeans might morph from skinny to wide-leg and the jacket might flit from tailored to peplum, but, as she puts it, 'the basic template remains the same'.

The reason for this disciplined approach is Massenet's demanding schedule: she is just back from Milan and is off to New York after our interview. 'I need to not think about what I'm wearing when I throw my clothes on in the morning,' she says. 'I have a seven-year-old and a one-year-old and I'd much rather be spending what precious time I have with them than trying on outfits. I'm low maintenance. It's probably a reaction against my childhood,' she laughs.

Massenet's grew up in Paris and Los Angeles. Her mother was a Chanel model during the 1970s; her father a journalist. 'My mother's aesthetic informed me the most,' she admits. (It's interesting to note that the Net-a-porter headquarters adheres to a Chanel palette of cream and black.) When Massenet was a child, her mother would have her designer friends custom-make mini versions of designer outfits for her daughter; while the young Massenet went to school in knee-high Cacharel boots, the rest of the children wore plimsolls. 'All I wanted was to dress like the other children, which probably explains why my approach to fashion is fairly low-key now.'

In a bid to fulfil the aspirations of both her parents, Massenet became a fashion journalist, first working at Women's Wear Daily and W in New York before becoming a fashion editor at Tatler in London. 'As a journalist I became adept at interpreting new trends and making them my own,' Massenet says. 'I would stick to my usual 'uniform' but pick a few pieces each season to bring it up to date. When grunge was in, the shirt was baggier, when androgyny was in, the jacket became more tailored. Usually these pieces would be vintage as I couldn't afford designer.'

Perhaps it's her elegant yet natural polish, perhaps it's the open face and feathery dark hair, or the country-club American accent, but Massenet bears a striking similarity to Jackie Onassis. She's one of Massenet's style icons, along with Katharine Hepburn and Audrey Hepburn, the latter of whom she was fortunate enough to meet. 'It was one of the highlights of my life,' she says. 'I was inspired by her incredible spirit and, of course, her fashion sensibility.' Massenet also reveals that when she was in her twenties, she was very thin - like Hepburn - and looked to the actress for style inspiration. 'The thing about those women is that they exercised restraint. They never did anything to excess. They are timeless,' she says. Funnily enough, that's just the word that describes Massenet.