David Miliband, the secretary of state, was nobbled on his way out of the loo. Professor David Coggon, chairman of a government advisory committee, was pinned down in a hotel bar. For those responsible for government policy on pesticides there has been no escape in recent years from Georgina Downs.

Now aged 35, she has for much of her life been exposed to a horrific cocktail of pesticides that was regularly sprayed on farmland beside her family home in Chichester, West Sussex. The spraying started when she was 11 and continues to this day - seven years on from when she first decided to devote herself full-time to making the countryside safer for the people who live in it.

Now, at last, she has reason to hope that she has made a difference. On Friday Downs won a ground-breaking legal case that represents probably the most significant setback for the agricultural chemicals industry in 50 years. The judicial review found that the government had failed to protect people, particularly rural residents, from exposure to pesticides. This judgment - against the Department for Environment, Food and Rural Affairs (Defra) - represents a significant breakthrough in a crusade that has already been compared with earlier campaigns to prove the health dangers of tobacco and asbestos.

Some farmers believe that the ruling may even lead to pesticides being banned – and that crop yields will fall significantly as a result (although the Soil Association says organic farming can actually produce greater yields).

Downs has now called on the government to ban spraying immediately near homes, schools and other public areas. The government is expected to appeal but for the moment she is enjoying her victory - the culmination of a campaign fought with extraordinary determination, despite appalling health problems.

She doesn’t like to talk about them, preferring to emphasise the importance of avoiding similar poisonings in future, but she has been in hospital for severe muscle wasting, leg pain and other chronic symptoms. Her father hinted darkly on the court steps that her health is worse than anyone realises; doctors say she has the brittle bones more usually found in a woman of 90.

For years the Downs family had no idea what was ailing her. Symptoms initially included blisters inside her mouth and throat; in 1991 her legs gave way. “I was absolutely devastated. I didn’t know what was going wrong. My body completely failed me,” she tells me.

Then, one day, she happened to look out of a window and noticed that the neighbouring farmer was spraying his crops. Wondering if this might be the cause, she asked him to give her family notice before he sprayed in future, but he typically gave only 10 minutes’ warning. (Bee-keepers, the judge drily noted, get 48 hours.) In any case, the sheer number of times that he sprayed his crops ruled out much chance of getting out of the way - last year’s salad crop alone was sprayed 30 times in six months.

So she made a complaint to the Health and Safety Executive. But although officials admitted privately that they wouldn’t like spraying to take place near their own homes, they were unable to prevent it because the farmer was acting within the law. The information distributed with pesticides urges farmworkers to wear safety equipment; but anybody else nearby is not deemed to be at risk.

At any rate, the only measure used to consider risk to anybody else was the spuriously scientific “bystander risk assessment”. This assumes that bystanders are exposed only briefly to one form of pesticide, just once.

On Friday the judge agreed that the “assessment” was inadequate and failed to take account of the hundreds of thousands of people - maybe millions - who live near fields that are routinely sprayed.

For what it’s worth, Downs emphasises that her campaign is not against farmers. Indeed, many of the thousands who have contacted her are farmers’ wives and children who have been exposed to the lethal chemicals.

In 2001 she realised that the only solution was to change the regulations. She had already given up her career in musical theatre because of ill health and returned to live with her parents after a spell in London. She gave her project a year - “That’s how naive I was,” she says. In the games room at her parents’ home, the green baize of the full-size snooker table is still covered by her paperwork.

As well as pursuing her legal case, she made a film showing a family of mannequins - including a pregnant woman and a baby - sitting in the garden as the adjacent field was sprayed, and another in which people living near sprayed fields described their symptoms. The list of ailments was extensive; indeed, Mr Justice Collins acknowledged that the film provided solid evidence to back up Downs’s case.

She methodically set out to lobby the experts and politicians who could help to change the regulations. Over the years she has seen four ministers and several senior advisers at Defra as well as innumerable MEPs and two European commissioners.

“When I first started attending conferences,” she says, “I made sure I stayed in the same hotel as many of the government advisers. Once, I managed to speak to the then chairman of the advisory committee on pesticides, Professor David Coggon, for two hours in a hotel bar. This led to him inviting me to make a crucial presentation to the committee, which was covered by the Today programme and got me my first meeting with ministers.”

Since then Coggon has done less to help; in fact, she accuses him of being one of those who are responsible for the poisoning of people like herself. “Professor Coggon informed me that he only saw 15 minutes of my two-hour film, the one the judge described as solid evidence, saying it was not a good use of his time.”

Junior ministers were generally supportive of Downs’s campaign, but getting policy changed required the support of the environment secretary. So she approached David Miliband, now the foreign secretary. “He turned me down twice in writing, so I went to a conference and got him coming out of the loo. I told him my name and said he’d refused to meet me twice and I wanted to know why.

“At first he looked confused but the third time I asked, he said, ‘Georgina, if there was a reason to meet you, I would have met you’ - or words to that effect. He was very arrogant.”

She had no greater success with Hilary Benn, the current secretary of state.

Despite her status as an outsider, a farming industry journal identified Downs recently as one of the 20 most influential people in the country. Over the years she has been given many awards. Dame Kelly Holmes, presenting one, described her as a British Erin Brockovich -– the self-taught American campaigner whose legal victory over polluters was made into a film starring Julia Roberts.

Even Downs’s family and friends must sometimes find her determination daunting. At the bottom of her press releases she writes: “The only contact for inquiries about this case is Georgina Downs.” Nor will she allow her father to answer questions on her behalf. If this smacks of control freakery, it’s easy to forgive. In her pursuit for justice she has given up her career and social life. Her friends call her “the Pesticide Nun” because she’s always cloistered - in summer, literally so: when the farmer is spraying, she and her father wear full protective gear just to cross the garden.

“But my friends have been great. They still call me and ask if I want to go out - and I still say no,” she says. There is no boyfriend in her life - “With all the work I’m doing, I don’t have a lot of time to look.” She did have a long-term relationship back in 1991, but then she fell ill.

“The other person didn’t want to hang around and quite rightly because he was only 18. I could hardly walk and the poor chap didn’t know what to do. If you make a checklist of what a man is looking for in a woman, it isn’t someone with chronic health problems running a big campaign against the government.”

Naturally, though, there have been moments when she has been tempted to give up her battle: “But then I’d look at the work that’s been involved and know that it would be a waste of all that time and effort if I gave up now. The other crucial thing that drives me on is knowing that I’m right. I don’t say that in an arrogant way. The evidence is really quite clear that the government has knowingly failed to act, has continued to shift the goalposts, cherry-picked the science to suit the desired outcome and misled the public.

“So I’ll continue to fight until the government starts to make the protection of public health the number one priority, instead of protecting industry interests.”

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