It can't offer the same high as cannabis, but hemp - a food so full of nutrients you could almost live on it - has one British supplier laughing all the way to the bank

One day two years ago Henry Braham found himself sat in the back of a car with Stephen Fry. As Fry's director of photography, the pair were in the middle of shooting the glittering costume drama Bright Young Things but, for a moment, Henry had rather different matters on his mind. He was on his mobile phone urgently discussing the price of hemp seed.

Braham and his wife Glynis Murray - who runs her own production company and has worked on award-winning commercials with Ridley Scott, David Bailey and Stephen Frears - lead double lives. During the week they're based in Notting Hill and are respected figures in the British film industry. On Friday evenings they climb into their green Land Rover, along with retired sheepdog, Bryn, and head for the village of Tawstock in north Devon.

As soon as they arrive at their 17th-century farmhouse, marzipan yellow with a thick thatched roof, the talk is less to do with shooting schedules and more about the hemp harvest. It's seven years since they started growing the crop - now pretty rare but once grown by every farmer in Britain under orders from Elizabeth I when it was needed to make rope for the navy - and six months since they began selling their home-grown hemp oil and toasted hemp seeds in supermarkets. Called 'Good Oil' and 'Good Seed', the products are an unlikely success story.

Say the word 'hemp' and most people think of hippies dressed in dungarees. And dope. The plant is a member of the cannabis sativa family, which marijuana also belongs to. What they're less likely to realise is that while hemp actually contains only trace elements of tetrahydrocannabinol (THC), the ingredient in dope that makes you stoned, it is packed with an amazing amount of fatty acids, minerals and proteins that are fantastically good for you (so much so, one nutritionist I mentioned it to said a diet of hemp seed and water alone would be fairly healthy).

It was the foot and mouth crisis that turned the Brahams onto hemp. They'd watched helplessly as their 150 disease-free sheep were led into the yard and slaughtered and wondered what the future might be for Collabear Farm, which they'd bought in 1996. They'd already experimented with less common cereal crops and discovered that hemp, once traditionally grown in the area, thrived in the warm, wet climate of the southwest. It was the beginning of a long learning curve. 'Everyone says don't be the first to person to do something. And that's absolutely true,' says Henry.

Home Office inspectors arrived to check that they weren't growing dope (but this didn't stop one over-zealous fan of Crimestoppers spotting the suspicious 10-foot tall plants and calling the police). Then the couple had to work out how to harvest the crop - it has incredibly tough stalks that can destroy the blades on a combine harvester. For the first two years everything that could go wrong did. 'One winter I can remember lying in bed with gales howling around the house and thinking I've got 75 per cent of the seed out there and it's literally blowing away,' remembers Henry, a softly spoken man, not given to exaggeration. The following year they experimented with a new way of getting the plant off the fields. 'It was a complete disaster.' They lost the entire crop.

But over time, and with the help of neighbouring farmers who welcomed the townies into the village, they got the hang of hemp. They worked out how to grow it, how to harvest it, how to dry it without it going rancid. Sometimes out in the fields combining at three in the morning, Henry confesses that he was a man obsessed. 'You had to solve it, didn't you love?' says Glynis. To begin with they concentrated on supplying the seed for the specialist bottles of hemp oil found in health shops. 'But then we realised that our quality was getting better and better,' says Henry. 'We were selling to big names who didn't care where the seed came from, didn't care about the taste. We thought - this is ridiculous.'

The answer was to turn away from health food shops and towards supermarkets. 'We came up with the idea of making hemp oil into a staple food product,' says Glynis. 'Turning it from a holier-than-thou medicine into an ingredient.' Marketing friends persuaded them to call the sweet, pine nutty cold-pressed liquid 'Good Oil' which had a hipper ring to it than hemp. When Henry's mother starting selling out at her local farmers' market in Wiltshire (and running down the stairs claiming her arthritis was much better) they knew they were in business. It was time to give Waitrose a call.

The Good Oil story would not have happened 10 years ago, even five. In a way it's the result of several 21st-century trends. First, a farming industry in crisis, forced to find different ways to make money (after a lifetime growing traditional cereal crops their neighbour, Francis Thorne, is trying hemp, too). Secondly, a savvy generation of consumers who know their hemp from their hash and are not alarmed by new ingredients. And finally, supermarkets that have realised consumers want to eat food that tastes good and is healthy.

Nutritionist Natalie Savona calls hemp 'a true superfood'. Uncooked, she explains, it contains the perfect ratio of omega fats 3, 6 and 9, which we don't produce naturally in the body but are crucial for healthy circulation, cell growth and the immune system. Traditionally the advice has been to eat lots of oily fish, but hemp is an excellent vegetarian alternative. It is said to help with eczema, asthma, heart disease, high blood pressure. There is even evidence to suggest that it can improve memory and relieve depression (parents of children with attention deficit hyperactivity disorder have noticed promising results). Small amounts of magnesium and zinc boost energy levels and regulate hormone balance. Plus there are the cosmetic advantages - Natalie estimates that incorporating it in your diet for two weeks will produce naturally moisturised skin.

Hugh Fearnley-Whittingstall has a bottle of Good Oil next to his cooker and uses it more than his traditional olive oil: drizzled on toast with a bit of salt in the morning; as a salad dressing with sweet tomatoes and salty cheese; it makes the perfect crisp and nutty roast potato. Although heating up the oil destroys some of the nutritional value, it still tastes good. 'It's got a distinctive taste, an acquired taste perhaps - like any strong olive oil. But like a good malt whisky you grow to love it.'

Does he think hemp's hippy reputation will put people off? 'I have to say that's never been a problem for me!' he says. 'I'm familiar with hemp in all its forms. Whenever I've cooked with, shall we say ingredients like hemp, in a cookie or a cake, it's tasted awful, but there have been other benefits! This is different. It's a culinary ingredient and unlike other novelty oils I think it will stand the test of time.'

Henry and Glynis are now planning to branch out into other hempy products - snack bars, seed-covered chocolate bars, even a dairy-free smoothie. Where once they farmed 40 acres, now they're renting local land and working on 750 acres. Remarkably, they manage to juggle filming schedules and the relentless farming calendar. 'Plus,' chips in Glynis, 'several homes and four kids.' Henry estimates they've invested about a million pounds in the business.

Their optimism seems to be well-founded. In New York the fashionable chef Denis Cicero uses hemp in his restaurant Galaxy Global Eatery (while drug laws prohibit cultivation in America, the seed can be imported). One item on the menu, hemp crostini with seaweed caviar, has proved so popular that Cicero is to launch a cookbook. All Henry and Glynis need now is a plotline on The Archers and we'll all be high on hemp.

Good Oil, £6.99, and Good Seed, £2.49, are available from branches of Waitrose, Harvey Nichols and Selfridges