High-street giants such as Marks & Spencer and Asos give thanks to the style council

Laura Chesters hears how a secret weapon is keeping fashion chains - such as Gap, Next, Abercrombie & Fitch, Tesco and River Island - ahead of the trends

How did Marks & Spencer know the pink winter coat would be a hit this season? How did Asos know to inundate its website selection with sweater dresses earlier this month?

Buyers and merchandisers have to use gut instinct and the data analysis of one woman – and her team of fashionistas – to make sure they don’t miss a trend.

Julie Harris, chief executive of WGSN, the fashion forecaster, is, in her own words, the “secret weapon” deployed by buyers. Harris’s team of fashion experts, analysts and data geeks flog their know-how to the likes of Gap, Next, Abercrombie & Fitch, Tesco, Puma and River Island, making sure the retailers don’t sell a dud line that could cost millions.

Harris says: “Retail has changed more in the last three years than it has in the last 30 years. The size of the mistakes that can happen when a buyer gets it wrong is staggering.”

Retailers are mocked when chief executives time and again blame poor sales on the weather. Selling coats in August or dragging out the summer shorts and bikinis during a cold May is asking for trouble.

However, this was the way it could go when retailers had to order months in advance. Now, spurred by fast-moving competitors online, the fashion multiples do constant “drops” of clothes, updating collections gradually rather than once for each season of the year.

Harris says: “Now it is eight to ten mini seasons. The consumer is in charge. There is no mystery behind the catwalks now. We don’t have to wait for [US Vogue editor] Anna Wintour to decide.”

Harris and WGSN sell their “maths and magic” to retailers. This is their compilation of data from websites combined with the instincts of fashion analysts, who spend their days with designers, alongside the catwalks and keeping a close eye on what fabrics are selling well.

Harris says that 44 per cent of retailers recently surveyed said they had been “late to market” on a particular line, potentially losing millions. Nearly 30 per cent said they had wrongly priced stock. These are problems that WGSN can help them to avoid.

Vicki Day, a retail expert and specialist in e-commerce, says that Harris’s team can make a bog difference to fashion empires that have the manpower to digest the data they provide. “WGSN is a fabulous tool for large retailers as it allows them to get in-depth research in so many areas,” she explains. “They can drill down and see micro trends and act on them quickly.

“Today everything is about immediacy, due to the internet.”

WGSN is owned by Top Right Group, the publisher once known as Emap. The group forked out £140m in 2005 for a business that began life as Worth Global Style Network in 1998, when it was founded by the entrepreneurial brothers Julian and Marc Worth. The pair have since left and set up a rival operation.

Harris has lived in Hong Kong and New York and is now based in London, but she travels frequently because geographical expansion is one route to snaffling big-name customers. The group is also looking at sectors outside clothing, so that subsidiaries focus on homeware and groceries. As Harris says: “There is plenty of space to grow and plenty to keep us occupied.”

Top Right is thought to want to sell WGSN eventually – a move that might seem counter-intuitive given the number of retail bosses who would rather avoid being a laughing stock when they’re reduced to arguing that they’ve been caught out by the British weather.