Between running a global beauty business, looking after five children and delivering lambs on her organic farm, it's a wonder Liz Earle has time to celebrate her success

She's got a few wrinkles, of course (she's 47 years old, with five children, one of whom is under a year old, for goodness sake). However, with minimal make-up, she has a natural glow, with apple cheeks and freshly blow-dried hair that's streaked with expensive-looking caramel highlights.

A classic English rose.

In fact, beside her, one might feel a bit like a Scottish pot plant that's been left out in the frost.

She's in the capital to meet her public, as John Lewis in Edinburgh have just released sales figures revealing that Liz Earle's "hero product", Cleanse & Polish, is the best selling item in the store. That's not just in the beauty department, but across every single category.

You could say that this eucalyptus-scented cream cleanser, which retails at 13.25, is a bit of a phenomenon.

However, that doesn't mean that she's resting on her laurels, in order to reap the rewards of a product that's sold around six million units worldwide (not to mention the rest of her ever-expanding range). Neither has she taken a back seat because Avon bought Liz Earle Naturally Active Skincare last year ("They bring an enormous amount of knowledge in challenging markets like North America, South America and China," she explains).

Instead, she continues to manage the 16-year-old brand, alongside her business partner, former John Frieda marketing executive, Kim Buckland, from their base on the Isle of Wight.

And it seems that she is now able to refocus her attention on certain aspects of the company she is most interested in.

"I started as a health and beauty editor 25 years ago, and quickly realised that it was the research that I loved more than anything," she explains. "Before Google, I used to sit in the British Library and that's when I developed my passion for finding out how ingredients worked."

Earle was also influenced by her father, who was an admiral in the Royal Navy.

"Because he was at sea quite a lot, his passion was his garden," she says. "He'd come home and spend every free moment gardening and, particularly, growing useful plants. He was a practical man, while my mum would only get a small patch of roses. I'd spend hours in the garden with him and that's what turned me on to plants and their properties - the functionality."

These days, part of her job allows her to operate like an alchemist cum 18th-century plant hunter - always on the look out for botanicals that might contain transformative properties. The brand is a "natural" one, in that they use plant extracts, but they're not always organic. "Sustainability", is their buzzword, explains Earle.

"There's an issue with some extracts," she explains. "Sandalwood, for example, has a fantastic smell, but if you cut down a 300-year-old tree to extract the oil, then it's not eco-friendly. If you can create it, molecularly identically, in a laboratory, then what's better for the planet? We have a broad team with specialist knowledge to answer these questions."

On her travels, she often photographs leaves and shrubs, then sends the images to Jennifer Hirsch, her resident ethnobotanist (a scientist who studies the relationship between people and plants), for identification.

"If you told me about some fantastic Scottish herb that was growing round the castle, I might be off there with my walking boots on," she laughs. "I used to send Jennifer pictures of leaves and things, and she'd ask how big they were, so I try to get my thumb in the corner of the picture as a scale."

The company is currently on the lookout for botanicals that have anti-inflammatory qualities, or contain a nutrient, such as chlorophyll or beta carotene. When she and her husband, photographer and film-maker Patrick Drummond, were on holiday in East Africa ("He's got wise to me," she says. "I'll say, 'let's go to Malawi', and he asks if it's a holiday or a field trip."), they came across the yangu tree, which is covered in fuchsia flowers. At that time they were doing lab research for their new conditioners, so Earle was straight on the phone to tell Hirsch about this plant, which they soon discovered was used by local tribespeople as a hair-shining treatment. It's now harvested ethically, as an ingredient in their Botanical Shine Shampoo.

The more cynical might imagine that it's helpful to have that kind of romantic back story.

However, there are other ingredients that are less glamorous, such as the fruit of the sausage tree, which is used in the Superskin Bust Treatment.

"It looks very strange, like a big salami," Earle explains. "If you're camping, don't pitch your tent under a sausage tree, as you could be killed by a falling pod. It's used by Malawian women, who don't wear bras, but - like women in the Western world - don't want to look saggy. So they make a poultice, which tones and tightens tissue."

However, despite the fact that her company produces firming products like these, Earle points out that they never use the term "anti-ageing" on packaging.

"I do feel quite strongly about that," she says. "I think society treats ageing like a disease that you have to recover from."

When I suggest that the beauty industry might also bully people into unnecessary worry about other skin issues, Earle is quick to counter that the media is to blame.

"I have to be very careful," she says. "I've got daughters, and I think as mothers we have to be very aware of the media that they are being exposed to. You can feel perfectly happy one minute, then you read a magazine and you've got the wrong kind of handbag and it can send you into a spiral of depression. Not me, personally, but potentially it could if you're young and vulnerable."

It seems that no company executive could be quite as sensible as Liz. Not only is she a businesswoman, avid researcher, mother of five, and able to carry off a body-con frock, she also finds time to pull her weight on the West Country organic farm where she lives with her husband and children. In fact, at the moment, she's gearing up towards a job that's as far removed from the cosmetics world as you can get.

"We'll be lambing soon," she explains. "It's very raw and real, there's life and death in the small hours, with creatures who are relying on you for their survival. Sometimes you can save a lamb and sometimes you can't. That's the harsh reality. You do your best."

• This article was first published in The Scotsman on 09/04/2011