It is hard not to feel a tad jealous of Safia Minney. Here’s a woman who set up one of the UK’s leading ethical fashion businesses, selling clothes to the likes of Sienna Miller and Minnie Driver. When not at her London house, she has a pied-à-terre in Tokyo, where — according to a relative whom I tracked down before meeting her — she has single-handedly transformed local perceptions about green issues. Don’t worry, he adds, she’s not the sort to spout dogma; she’s incredibly warm and works on a human level.

And guess what? She’s a gorgeous slip of a thing, with huge brown eyes and unusual colouring, the legacy of a Mauritian father and Swiss mother. Throw in a wealthy banker husband and two well-adjusted children, Natalie, 10, and Jerome, 13, and you get the picture. Which is why, when we meet at her East London office, I’m on the lookout for signs of weakness. Cruel, I know, but an SUV parked round the back would be just the ticket.

No such luck. Minney lives and breathes People Tree, the fashion label she set up to promote social and environmental awareness. Sipping tea and looking remarkably cool in the heat, she is wearing a turquoise organic cotton tank top from the summer collection and beads strung by deaf and mute women on a People Tree project in Calcutta.

She speaks in short bursts, darting off at tangents and losing track of the original question. So overflowing with tales of injustice being done by “fast” (mainstream) fashion is she, it’s tough finding out about her. I want to know what motivates her, how she copes with clashes between lifestyle and ideals, family life and the daily struggles that knock the stuffing out of the rest of us.

People Tree is the brand you are most likely to have heard of if you are remotely interested in clothes with a conscience. Its organic, often Fairtrade-certified garments promote natural and organic farming, which is safe for the growers as well as the environment. It campaigns against the use of pesticides which, according to the World Health Organisation, kill 20,000 cotton workers every year (see box).

This September People Tree celebrates five years in the UK, having gone from flogging T-shirts and casual wear to “handicrafts types” in their forties, to selling smarter, fussier pieces — such as this season’s silk dresses — to people half that age. Last year its annual turnover was up 40 per cent on the year before. It has sold to Selfridges and it recently doubled the size of its stand in Topshop’s flagship store in Central London, and there are plans to enter an undisclosed high street chain next year. All of which are significant milestones for ethical fashion. Mostly, though, customers buy online.

But it hasn’t always been simple for Minney, 42. She started People Tree from her home in Toyko 11 years ago, led there by her husband’s work. It was, she says in her soft, almost childlike, voice, a deeply hostile environment. “I wanted to carry on buying organic food, being vegetarian, shopping for clothes in second-hand shops and recycling. But Japan is a very corporate, consumerist society. No one challenges it; there’s a stigma attached to any kind of green work. Dolphin meat was once put through my letterbox to scare me. People didn’t like me stirring things up.”

But with the help of a group of students, she gradually built up a loyal following. In 2001 she expanded the business to the UK and found that doing the same thing here was no easier. Shoppers were starting to get to grips with fair trade and organic food, but the concept of an ecologically sound jumper was still radical. Educating the consumer, she says, is the No 1 challenge that faces People Tree.

She is frustrated that too few people understand the difference between Fairtrade fashion and ethical fashion. The latter is about meeting legal minimums such as the International Labour Organisation’s convention on labour rights and basic health-and-safety regulations, she explains patiently, and no doubt for the umpteenth time. Fairtrade, what People Tree is all about, goes a step farther, using fashion as a tool for social development and to provide an income for marginalised communities.

You get the sense that Minney knows what’s good for us, which some might find off-putting. She pauses while we’re talking to ask whether going into the nitty-gritty of water polluted by arsenic in Bangladesh (where the garment industry has hogged clean water supplies) is a bad idea, saying: “Will that make people cringe?” But she doesn’t skirt the issues. “Fashion is political; that’s got to be part of the reason why people buy the clothes,” she says. “When I launched, I’d tread softly; now I’m much more straightforward. I’ve seen the miserable conditions people live in, making the products that we buy on the high street. Now I say: if you buy a dress from People Tree, you do so in the knowledge that you are helping to distribute wealth more widely around the world.”

Minney believes that a growing number of people are turned on, not off, by this. She doesn’t go as far as to say that the clothes aren’t as stylish as they could be, but she agrees that people fall in love with them after, not before, they put them on. “The feel of the clothes is a strong point; we don’t use synthetic materials, so people are rediscovering hand-embroidered fabrics and organic cotton,” she says.

Minney grew up in Berkshire, leaving school at 17 with no A levels to take up a publishing job in London. She came relatively late to campaigning, not the eco-tot you might imagine, waving banners from an early age. “My father was a petrochemical scientist; my mother worked in publishing, so taking an ethical route into business was not expected.”

It wasn’t until she discovered an Oxfam shop close to her job at Creative Review magazine, where she’d browse in lunch hours, that she started to think about the role that fashion plays in global trade. Then, at 25, she upped sticks, marrying her boyfriend James, an investment banker, and following him to Tokyo. The irony of being married to a banker is not lost on her. “He is the epitome of everything I shouldn’t be involved with,” she whispers, with a girlish giggle. “I like to think that the only reason he keeps his job in banking is to support People Tree.”