Behind the purple door on the cobbled streets of Alston, Cumbria, the cooking pot was seldom off the boil. In the ordinary kitchen tucked high in the North Pennines, thick brown liquid was poured into moulds imported from Belgian chocolatiers. Luxurious bars of high-cocoa-fat chocolate were wrapped and labelled and placed in jiffy bags, then posted to all corners of the country.

It would have been an uncomplicated and charming cottage industry, but for one special ingredient: each 150g bar contained up to 3.5g of cannabis. For six years, its presence in the chocolate helped ease pain for more than 1,600 people with multiple sclerosis, almost 2% of all British sufferers. It also caused the chocolate's distributors, Mark and Lezley Gibson, to fall victim to village gossip, police raids and legal action that has left them branded as drug dealers.

On Friday, a jury found the Gibsons, and an associate, Marcus Davies, guilty of two counts of conspiracy to supply cannabis between 2004 and 2005. There were tears and gasps of shock in the public gallery at Carlisle crown court when the decision was delivered. For Lezley, who suffers from MS, the decision at least ended the mental and physical agony of a prosecution that has dragged on for nearly two years. But the reverberations of the decision reach far beyond the Gibsons' kitchen in Alston. The verdict is discomforting for those who assumed this country had, in effect, decriminalised cannabis. And it is painful for those MS sufferers who have suddenly found their supply of cannabis chocolate cut off.

The Gibsons' journey to Carlisle crown court began back in the mid-80s. Lezley was a trainee hairdresser, good enough to beat the likes of Andrew Collinge and Nicky Clarke in apprenticeship competitions. While the men went on to become celebrity coiffeurs, however, the worsening pins and needles in Lezley's legs took her to hospital in Carlisle, where she was diagnosed with MS. Treated for 10 weeks, she had two agonising lumbar punctures and was given steroids, which caused her to grow a beard and double her weight to more than 14 stone. The final piece of "medical" advice before she was discharged was, she says, to forgo butter and eat margarine instead. "I was called in-valid," she recalls, emphasising the word. "I was disabled. I was written off, no longer any use to anyone. When I was diagnosed, they said I would be incontinent and in a wheelchair within five years."

Twenty-two years on, Lezley is a slight, attractive woman with a warm smile, and no wheelchair. For three years after her diagnosis, however, she suffered paralysis of her right and left sides and periodically lost the power of speech and sight in her right eye. "This was the normal downward spiral of MS. Then I met Mark and started using cannabis," she says. Like many young men, Mark was a recreational user. Lezley noticed that when she smoked it with him, she felt better.

"I read Charles Kingsley's The Water Babies when I was young and I've always been Mrs Doasyouwouldbedoneby," she says. "Once I found out that cannabis did this to me, I couldn't keep it quiet. I wanted to help other people." The former MP Robert Kilroy-Silk inadvertently assisted Lezley when she appeared on his show to talk about MS in the mid-90s. There she met other MS sufferers and was inspired to found a support group, Therapeutic Help from Cannabis (THC). Fortified by a growing body of scientific evidence about the medicinal effects of cannabis on MS, Mark got involved and travelled to universities to talk at conferences about the drug.

It was when the Gibsons began talking publicly about cannabis that the police took an interest. The couple were first raided in 1989: Mark spent a week on remand in Durham prison. They were busted again in 1995 and again in 1999. In 2000, Lezley was acquitted of possession on the grounds of medical "necessity".

A friend and fellow MS sufferer, Biz Ivol, began producing "Cannachoc" for a neighbour with MS. She, too, was arrested and prosecuted. When her illness was exacerbated by legal action, the Gibsons offered to take over the production of the chocolate in 2000. When Ivol died in 2004, they named it "Canna-Biz", in her honour. Qualified in food hygiene, Mark began making the chocolate for about half a dozen MS sufferers in the kitchen above his gift shop in Alston, where he sells new age crystals and ethnic ornaments.

"It felt good to be of some use, to help society. It was terrible to be told you're no use at 20," says Lezley. As word spread, people wrote letters addressed to "The lady with MS, Alston" or turned up at their door in wheelchairs. Those who found cannabis helped reduce muscle spasm and increased bladder control were relieved to find an alternative to giving £150 a week to a drug dealer. Soon the couple were posting bars to more than 100 people a week.

As long as he didn't sell it, and only gave it to MS sufferers, Mark believed he had a defence in law. If prosecuted, he thought he could deploy the defence of "necessity", which allowed for an illegal act to avert greater harm (or, in this case, pain). A number of people, including founders of medical marijuana cooperatives and even a GP, Dr Anne Biezanek, who supplied her sick daughter, have been acquitted of possession and supply using this medical "necessity" argument.

Gibson was meticulous about how he produced and supplied Canna-Biz. He obtained the cannabis for free from altruistic growers and bought the chocolate (ideally, organic Green & Black's; otherwise Tesco's or Morrisons' own brands). Lezley helped with the administration and they instructed users to take no more than three squares from the 24-square bar each day to ensure no psychoactive effects. They never distributed it through their shop. Crucially, they never sold it, instead suggesting a donation of between £1.50 and £5 to cover their costs (some MS patients would give more; many chose never to pay anything). And they insisted that any "patient" must send them a doctor's letter, on headed notepaper, confirming they had MS. Lezley says the notes showed that almost all their patients were over 40: "It's not people looking for a cheap thrill; it's middle-aged people who are ill."

Curtains began to twitch as Mark strolled across the street to Alston post office every day with jiffy bags of cannabis chocolate. Small-town gossips reckoned the 4x4-driving couple must be raking it in. Mark was summoned to meet a senior Cumbria police officer in 2002. He was warned: open a cannabis cafe and you'll be shut down. But police advice about their chocolate operation was more ambiguous. Mark told the court it was suggested he should not be "quite so blatant" about it. "He didn't tell me to stop," says Gibson. "To this very day nobody has told us to stop."

Nevertheless, heeding police warnings, the couple reduced their media profile and began using a regional post office instead of their local one. Davies, an associate from Cambridgeshire, built a website for their new not-for-profit scheme, Therapeutic Help from Cannabis for Multiple Sclerosis (THC4MS). He also provided them with a PO box address in Huntingdon; it meant the couple's home address was no longer publicised. When their local bank refused to let them open an account for THC4MS, Davies, who is registered disabled and grew his own cannabis to treat his severe diabetes and epilepsy, agreed to cash donor cheques for the Gibsons.

Internal police documents read out in court showed that Cumbria police decided it would be "oppressive and vindictive" to mount a surveillance operation on the couple after Lezley was acquitted in 2000. Early one morning in February last year, however, the police knocked on the Gibsons' purple door. Bars of Canna-Biz had spilled out of a jiffy bag at the Post Office and the police were called. Along with Davies, they were charged with conspiracy to supply cannabis.

Legal arguments dragged on for nearly two years - destroying Gibson's gift shop business - before their case finally opened in court two weeks ago. Jeremy Grout-Smith, for the prosecution, argued that while the couple were not conventional drug dealers, there was no defence in law. "To supply cannabis, even if you believe it is doing good, is not a defence," he said. Police found details of three bank accounts at Davies' home into which more than £39,000 worth of cheques had been paid between March 2003 and March 2005. "So this seems to be distribution on quite a large scale and, to some extent at least, the defendants may have benefited financially - although the Crown does not claim this was their main motivation," he said.

The Gibsons and Davies claimed that most of the sum was the income from other members of Davies' family. Even if some was money from donations, they said it was all ploughed back into the not-for-profit Canna-Biz operation. In six years they have supplied more than 33,000 bars and calculate they have given away cannabis with a street value of £500,000. The pair are not visibly wealthy. "We're not very good 'drug dealers', are we?" says Lezley. "If I'd sold it, I wouldn't be sitting here. I'd be in Spain with the rest of 'em."

As the arguments were trotted out during nine days in court, little indignities struck home. On one morning proceedings were delayed because the prosecution barrister had a medical complaint. Lezley, who struggled to walk into court and found it painful to sit down for more than 10 minutes, was on time every day. She believes her previous court battle advanced her MS by five years. "At the moment I don't know if I'm able to move when I wake up," she said two days before the verdict. "I'm not sleeping, I'm constantly in pain across my shoulders. I'm not taking as much cannabis as I should because I'm stuck in court."

The couple struggled to retain their composure when faced with legal professionals who required correcting on several basic facts about medicinal cannabis and the law. When Grout-Smith asserted that no one had been acquitted of supplying cannabis on grounds of medical necessity, he was put right by Mark. The judge struggled to understand how a commercial medical cannabis treatment for MS users in Britain, called Sativex, could be unlicensed yet still be legal in the UK. Mark, again, was on hand to explain: in a government fudge, it was denied a licence in Britain but for the past 12 months has been made available on a "named patient" prescription basis. A doctor must get a licence from the Home Office to prescribe it to individuals. A survey by Disability Now found few patients have been able to get it because the process is so bureaucratic and expensive. Lezley is now licensed to carry a Sativex spray but finds smoking cannabis better. The spray is "very, very strong" and, she says, more likely to incapacitate her than smoking.

The Gibsons are not the sort of campaigners who crave courtroom publicity. But their case made public some interesting facts. The couple kept 1,036 letters from doctors confirming patients had MS. Of those, 65 made specific reference to cannabis chocolate, showing that some doctors had full knowledge of what they were writing the letter for. One MS sufferer, Helen Wallace, gave evidence from her wheelchair in court that she had told her doctor precisely why she needed the note. In this way, many of the letters from the medical profession were de facto prescriptions.

Despite such support, the political climate has shifted. The government is determined not to be seen as "soft" on the drug since it was downgraded from class B to C. (Paradoxically, downgrading cannabis has damaged the Gibsons because recreational users who once donated money or campaigned for their medical cause have dropped away, content that they can smoke a spliff in peace.) As well as a growing body of research showing its medicinal applications, there is, equally, more scientific evidence linking cannabis to mental illnesses such as psychosis and schizophrenia. The studies agree on one thing: cannabis affects different people in different ways.

Three months after the Gibsons were arrested, legal opinion also turned against them. While several cannabis suppliers successfully evoked the defence of medical "necessity", the court of appeal ruled in May 2005 that necessity could not be a defence in six test cases of supplying cannabis. Removing this common-law defence has, in effect, recriminalised the medicinal use of cannabis. Lawrence Wood, chief executive of the Multiple Sclerosis Resource Centre charity, says the law is "an ass" and society is hypocritical to allow possession for personal use without explaining where it can be obtained safely. "If the government had licensed Sativex, and made it easily available, then these people would not have needed to supply it in this way," he says.

Since the verdict, the couple have received emails from recipients of their chocolate. "I shall miss it very much," says one woman with MS. "I have tried gradually using less each day but was in so much pain I started back on a full dose. Don't know what I shall do when this bar has finished."

Lezley is defiant. "The prosecutor told me what I did was wrong but the law is wrong. It's evil and cruel and totally unfair. As a person who is ill, why am I in court? It can't be a crime to want to be well. If it was paint stripper, I'd take it. It's just unfortunate it's illegal. I'm sorry that cannabis makes me well and I'm sorry I'm going to keep taking it, because I don't want to be in a wheelchair and I don't want to be incontinent."

The maximum penalty for the unauthorised supply of cannabis is 14 years in jail but the pair, who will be sentenced in January, have been told they will not face a prison term. They know, however, they must pour away the last Canna-Biz and pack up their pans. They have no defence in law; MS sufferers will no longer receive cannabis chocolate pain relief through the post. "I want the judge to send a letter to all those people saying why they will no longer receive our medication," says Lezley. "I can't do that".