A £25 must-have frock. The first high-street sari. The 'moob tube'. Has Asda really become the store that dresses the nation? Holly Williams reports

It might look like your average little black dress, but the 'tummy-tamer' has designs on world domination. The dress, which went on sale last month, sold out within hours, and has since been spotted going for twice its retail price on eBay. The Bridget Jones conundrum – that the wearing of giant support knickers might help you get your object of desire into bed, but may prompt them to leap straight back out again when it comes to the big reveal – is done away with: support is zipped into the garment, promising you an hourglass figure.

And the design genius behind this clever piece of engineering? Asda. That's right, the supermarket. And the price? A recession-friendly £25. And the person behind the smarter little black dress? Fiona Lambert, who as brand manager at George for Asda, is all about making supermarket fashion the best way to shop. Those who would never even think of tossing a dress in their trolley along with a bag of pasta and a bunch of bananas might want to reconsider.

"I was inspired by how you feel when you get up in the morning, how good you feel when you pop on a great dress," explains Lambert, talking about the tummy-tamer. "It took a year to design – there was nothing that did what it does."

But she confesses to being taken by surprise at the massive response it prompted: "We realised the market was there for these products, but not just how big it was. It caught us out a bit." But fear not: there will be more tummy-tamer dresses out in March, plus an extended range in July.

The 46-year-old Lambert was with George at its beginning, back in 1989: she had started her career as a designer and pattern cutter for Next, and when designer George Davies moved to set up a clothing line for Asda, she followed. She's switched between Next and Asda throughout her career, but returned to George as brand director in 2007, absolutely determined to make her mark. And she has: last October, Asda heard they had wrested the title of number one clothing retailer, by volume, from Marks & Spencer. It seems Lambert is set to snaffle the title of "woman who dresses the nation" from M&S's Kate Bostock. The key to her success has been to ditch the dodgy bargain basics and improve the quality, while also coming up with some unusual products that'll tempt shoppers out of the fruit'n'veg aisle and into fashion.

"None of us buy something because it's cheap – it's got to be the right product. When I first worked at George, it was a real market-leading company, and when I came back I felt that had been lost. I wanted to bring the innovation back," Lambert explains.

So Lambert launched a series of firsts: an Asian range, with traditional garments at rock-bottom prices; a wedding package, including a bride's and two bridesmaid's dresses and the groom's suit, for under £200; and the popular in-between jeans in sizes 11, 13 and 15.

"Our customers are very savvy," she enthuses. "Improving the fit of our clothes was really important, especially in the sort of environment where they maybe don't get to try the clothes on, because they've got a trolley or their kids with them."

The innovations are not all for women – another surprise hit was the men's body sculpt vest, or "moob tube" as it became known. Marketed as the "£7 six pack", Asda.com sold out in just four minutes. "Men have insecurities as well. The vest was incredibly popular, and we had about a thousand people on the waiting list – it's back in store this week."

Lambert isn't just a businesswoman: she has a real enthusiasm for clothes. She decided when she was 11 that she would work in fashion, and bagged a job at Next after completing a degree in fashion at Trent Polytechnic, where her thesis was on the psychology of fashion. And she's still pretty sure she knows what makes shoppers tick: "One of my strengths is that element of customer understanding, an intuition about what they want. But we also talk to our customers a lot."

Supermarket fashion is even more competitive than the high street. "Our customers give us five minutes of their time. I have to edit, with my team, so that our products are really, really what the customers want," says Lambert.

Having children has helped, as a lot of George's customers are mums. Alongside her team of 72 designers, Lambert now also has an adviser closer to home. Her daughter is 16 and Lambert takes her shopping: "She gives her opinions probably more loudly than other customers!"

Asked how she manages to juggle her job and children, Lambert explains: "They keep you incredibly grounded. I'm very lucky – my job is my hobby. I get to be creative at work, so I'm free for them when I get home." George's managing director, Anthony Thompson, is leaving the company and Lambert had been tipped as a possible replacement. But she expresses no desire to leave her current position.

"Being a brand director is a great job – I love being so close to the customers and the clothes." Good news for Asda shoppers, who can carry on stocking their closets as well as their cupboards.

Super women: The high-street heroes

Kate Bostock, Marks & Spencer

A down to earth everywoman – who also cut her teeth at George – Bostock helped turn M&S's fortunes around when she joined in 2004. She introduced the British institution to the concept of fast-fashion, making it a place to buy more than socks and pants.

Jane Shepherdson, Whistles

Having turned Topshop into the coolest place on the high street, in 2008 Shepherdson bought a stake in Whistles, and set about reinvigorating the middle-market, boutique-style chain.

Belinda Earl, Jaeger

After holding the reins at Debenhams for 18 years, the experienced Earl joined Jaeger in 2004, developing a more exclusive brand image.

Yasmin Yusuf, Miss Selfridge

Having made a name for herself at M&S, Yusuf became creative director at Miss Selfridge in 2006, and tried to turn its fortunes around. She succeeded, bringing the company out of the red and its garments straight into the wardrobes of fashionable young shoppers.