The woman responsible for turning the high street into a maelstrom of consumerism at regular intervals does not seem the excitable type. Apart from her dark bob, which is more Harriet Harman than Anna Wintour, and a weakness for wearing black, Margareta van den Bosch, H&M’s (increasingly not-so) secret weapon and the woman behind the chain’s phenomenally successful designer collaborations, could almost be a mid-level bank manager. Maybe it’s because, now that she is 65, with a 34-year-old son and a design team several decades younger than she is, stateliness is the most dignified option.

She was probably always the phlegmatic sort. At 42, she approached H&M with the suggestion that they should make her their head of design. Which they did. “I had 22 years’ experience, working for all kinds of labels in Italy,” she points out. “I thought they could use someone like me.”

In those days, H&M’s design team consisted of seven people. Now there are more than 100, based at the company’s HQ in Stockholm and fiercely democratic. “We decide together on the designers we want to work with,” says Van den Bosch (her surname comes from the Dutchman she married while working in Italy, although they parted when she returned to Sweden). “I should say, however, that the idea for the first one came from the marketing department, which was looking for a way to drive new customers into the store.”

That particular mission was accomplished, I think we can agree. Five years ago the company ruffled its rivals’ feathers by launching a one-off collection designed by Karl Lagerfeld (Van den Bosch put together the programme), key pieces of which sold out within 48 hours. Then came further successful collaborations with Stella McCartney, Roberto Cavalli and Viktor & Rolf.

In a few weeks’ time the French catwalk doyenne Sonia Rykiel will join the roster of illustrious names — and from 9am on Saturday, November 14, H&M’s Jimmy Choo collaboration, one of its most feverishly anticipated link-ups yet, will be available in 19 stores across the UK and Ireland. It is safe to assume that by 9.10am, each of those branches will look as if it has been hit by a tornado.

Alliances between designers and the high street are not new but, until Lagerfeld, they had not been conducted with such internationally renowned names. H&M’s vaunting ambition, enormous manufacturing clout, enviable marketing budgets (the Lagerfeld project was launched with a two-minute global television campaign) and faith in the fashion literacy of its customers (last year it hooked up with Comme des Garçons, a label worshipped by cognoscenti but less familiar to the average high street consumer) turned these marriages into much-hyped events.

Such is the current excitement about Choo — a label famous in every country with access to episodes of Sex and the City — that customers will be restricted to a ten-minute shopping slot.

The Choo liaison genuinely breaks new ground in that some of the products (a leopard-print clutch, for instance) are remarkably similar to Jimmy Choo’s own range. The tie-up also marks Choo’s first foray into clothes. Prices are high for H&M — about £70 for a pair of shoes and £170 for the thigh-high boots, most of which are made in Portugal — but that is a sixth of what Choo devotees have paid in the past.

Isn’t there a danger, for Choo, that those fans will now come to H&M instead? “I think there is,” says Van den Bosch, looking far from broken-hearted. “But in return Jimmy Choo gets incredible exposure. His name may be familiar to a biggish audience but in Sweden for instance, you can’t buy the real thing.”

It is no secret that some products in the designer ranges have been loss leaders. Because of relatively high manufacturing costs, designer collections account for a tiny fraction of H&M’s £8.5 million turnover. But that is not the point. “The Viktor & Rolf wedding dress cost a lot more to produce than we charged for it,” says Van den Bosch, “but we like the rumours around these collections. They create a sense of excitement about shopping.”

That is putting it mildly — as designers know, which explains why no one has yet rebuffed an overture from H&M. “Karl didn’t hesitate when we approached him,” says Van den Bosch, “although he took his time putting a collection together. But he is incredibly busy — and his approval opened every other door.”

Perhaps it is slightly puzzling that H&M should have turned out to be so visionary and so successful, with 1,800 branches worldwide and profits that continue to rise (up last year by 14 per cent). Stockholm is not a particularly fashion-conscious city. “London is the most fashion-aware city in the world today,” says Van den Bosch, “along with Tokyo. New York is quite conservative by comparison. Moscow likes lots of glamour and Saudi Arabia wants underwear. But although you can’t get some labels in Sweden, there is much more interest in fashion there than there was two decades ago. I’d say that is true of almost everywhere.”

Keeping tabs on what teenagers and twentysomethings — H&M’s core market — want requires strategic planning from a woman who does not wear the latest trends herself and likes to shop for vintage. Her method relies on “a young team and constant observation”, she says, “and by that I don’t just mean looking at fashion magazines. You have to watch normal people all the time and be part of contemporary culture, celebrity culture. I don’t have the patience for Big Brother or blogs but there are people in my team who do.

“In some ways fashion hasn’t changed that much. It’s just that young women show more flesh — and there’s a lot more plastic surgery.”

She agrees that it is often hard for women over size 14 and over 50 to find well-designed clothes on the high street. “We go up to a 46 (UK size 20) in some ranges,” she says, “but it’s true that you need confidence and a certain fashion awareness to feel comfortable shopping in our stores.”

About 18 months ago, Van den Bosch decided to slow down. Now holding the title of creative adviser, she has scaled back her days from 11 hours to six hours, allowing her to spend more time at her apartment on Lidingö, an island outside Central Stockholm, and to see more exhibitions and concerts.

Given her rather ruminative demeanour, I wonder how she feels about encouraging a disposable attitude towards clothes. She looks momentarily fazed. “I don’t think that’s what we do,” she says eventually. “We are trying to offer value for money. We are a very self-critical company. What we are really providing is basics but with a fashion feel. Our ambition definitely isn’t to make clothes that are thrown away.”

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