Everyone who strays into Sally Ambrose's sightline is potential research for her job. She'll clock the colour of their handbag, the cut of a collar, the height of a heel and the length of any earrings. And when she lurks among the rails of Marks & Spencer she scrutinises which tops in which colour are vanishing most swiftly and which necklaces most readily bring browsers to a halt.

As head of buying for footwear and accessories at M&S it's Ambrose's job to anticipate what customers want even before they know they want it. "It's a question of marrying buying history – what's always sold well – with new trends," she says. "You have to steer the design so that you're taking risks with a new look while keeping a bankable style."

The power to impose a personal vision on the rails of one of Britain's most familiar fashion chains is something that many high street shoppers would covet since almost everyone has opinions on what the stores should and shouldn't be selling. Usually these opinions are based on personal preferences – which have to be instantly jettisoned for a career in fashion buying. "I've bought in lots of things across the years that wouldn't suit my taste," says Ambrose. "You have got to understand the customer you are selling to and be objective." Ambrose, 44, veered into a fashion career more by accident than design.

At school, sport was her passion and she chose a sociology degree at Exeter University. "I did my dissertation on advertising, so was steering my course in a business direction," she says. "But I was flirting with marketing and sales jobs when I graduated."

It was a series of holiday jobs at her local Jaeger store that fuelled her interest in fashion – "I never wanted to be a designer but I'd always loved watching trends and influences" – and, on a chance visit to the Jaeger branch where she had temped, she decided to heed the manager's suggestion to apply for the company's graduate training scheme in retail management.

"During my holiday jobs as a sales assistant I'd be sent to lots of different retail environments so I learned all the different store profiles and customer types, which helped me through the rigorous selection process," she says.

She became assistant manager of Jaeger's Harrods concession and discovered that her sports passion served her unexpectedly well on the shop floor. "Competitive games give you the ability to pursue a goal, to drive a team and to believe in yourself, which is handy when you're having to persuade a feisty sales assistant who works on commission to empty a box of hangers instead of chasing the next customer."

Ambrose moved into the buying office after a chance meeting with Jaeger's head of design. "As part of my management training scheme I'd had to develop and sell a scent," says Ambrose. "I won the exercise and reformed my ambition to get into buying." As assistant buyer she learned how to put a collection together – bridging the departments of buying and design to ensure that new launches were commercially viable.

"Design would present their ideas and we might tell them they were missing a key v-neck or a blazer that's always sold well, or we might identify a need to put coats together earlier than usual because so many had sold to tourists," she says. After two years in the buying office, she moved to M&S as a trainee buyer because of lack of promotional opportunities at Jaeger.

Predictably she began in knicker packs – helping steer the choice of trims, cuts, patterns and fabric bases – and briefly panicked at the shift from high-end fashion to co-ordinating the nation's smalls. She remained in lingerie for five years. "When you are a buyer in charge of a range it's huge," she says. "My first responsibility was buying boxed bras with a £22m budget and suddenly every male friend I had was interested in what I did and the difference between plunge and balcony bras."

She then moved via formal trousers to casual tops: "I used to get on a plane to Turkey where our manufacturers were then based, trial key shapes, put them in the stores in January, look at what had sold best in February then get back on a plane to Turkey to plan the rest of the season which would hit the shelves by April." Manufacturing has now moved to China and India, which Ambrose, who manages six assistant buyers, visits a couple of times a year in between visiting M&S's across the country and lurking in rival stores to assess what customers are buying.

Since her graduation in sociology, numerous retail-oriented degrees have been introduced, but, although a useful leg-up into a buying career, formal fashion and retail qualifications are not a prerequisite. "Our graduate scheme looks for relevant experience, which could be stints working as a sales assistant, as I did, provided you can demonstrate how you can sell well to customers and understand trend forecasting.'

Her A-levels in art proved useful – "I can sketch out how I want a lapel or a heel to look,' and she credits an economics A-level with instilling in her crucial business sense. But the core skill, which can be gained by attentive long-term loitering in any high street, is to know the customer. "It can be a very pressurised environment because you can have buyers and designers not seeing eye to eye and you have to call the shots," she says. 'You have to have the confidence to fight for what you believe in and go with your gut instinct whether it's a belt or a bra strap.'

The Guardian 27/2/2014

Anna Tims