But in the end she agrees, reluctantly, "if it will help Mark". And the story that Margaret has to tell is of pressures that would have sent a less courageous woman packing long ago. For in the 13 years since Mark began gathering evidence of the effects of OPs on both animal and human nervous systems, Margaret has not only had five children, but has also coped with cut telephone lines, threatening intruders, buildings that mysteriously fall down or burn down, shots being fired at the farm - and more.
The Purdeys live in a converted barn on a windy Somerset hillside: heaven in summer, hell in winter. They have no mains electricity (although a generator kicks in at night); their youngest, Tansy, is still in terry nappies; and until she was born two years ago the whole family lived in a caravan without so much as a washing machine.
It's a far cry from Manchester, where Margaret grew up with expectations ranging all the way from a job in Woolies to getting married at 16. But this wry, delicate-looking woman is, as Mark puts it, "as strong as steel". Having set her heart on being a farm worker she ended up in the West Country with Mark - who already had two children by a previous marriage - pulling swedes and milking cows.
Then, in 1984, they defied government instructions to dose their Jersey herd with warble fly treatment. "This is a systemic organophosphate which is poured along the cow's spine, seeps through the skin and changes the entire internal environment of the cow into a poisonous medium in order to kill off any insects," Mark claims. "But a child could see the stupidity of doing that with a chemical derived from a military nerve gas of the same type used by Saddam Hussein in Iraq".
The Purdeys were young idealists who did not expect to succeed, but, much to their own surprise, the case went to the High Court and they won. Following international publicity, Mark was inundated with letters from farmers who suspected - but could not prove - that they were suffering symptoms of OP poisoning.
Fired with the belief that the truth about OPs was being concealed from the public, Mark - who has no formal scientific training - turned himself into an expert on OPs and nervous diseases, lecturing to bodies such as the Medical Research Council's toxicology unit, and the Edinburgh International Science Festival.
Not only were these chemicals causing sickness, depression, fatigue, spasms and weakness in farmers, argued Mark, but they were also the root cause of BSE in cattle: "Particularly high doses were used in the UK of one type of OP containing phthalimide," he claims, "which is chemically related to thalidomide."
Mark worked - and still works - long into the night and without pay to gather evidence and develop his theory. He admits he is obsessive. Certainly, he is an idealist - a highly intelligent and convincing one.
But the price of his campaign, for the whole family, has been high - both personally and financially. "We lost a lot of money through campaigning", says Margaret, and mentions pounds 100,000-worth of milk quotas for which she thinks they could have been eligible, had Mark's energies not been totally focused on OPs.
From the very start they were branded as troublemakers by many in the local community. And being organic farmers who were outspokenly green and visibly broke made them a natural target of more conventional colleagues. "People said we shouldn't be allowed to farm, and that we used to walk around in the nude," smiles Margaret. "In fact, we used to bathe in the dairy because we have no bathroom, so someone must have been spying on us."
Did the hostility upset her? Margaret grins, and says: "You get used to it." For she is a woman who just gets on with it, as when she drove to her antenatal appointment on a tractor because the couple could not afford to insure their car. Parking at the hospital was a bit embarrassing, she concedes, "but they didn't clamp me."
They have run out of cash at times, Mark having spent the last of it on photocopying. During one particularly thin time, the children told a friend. "She bought pounds 20-worth of groceries and said, 'pay us back another time'," says Margaret.
As for clothes, "we don't have any," jokes Margaret, while admitting that at 40 she is getting a bit fed up with "sharing the good jumper with Mark. It's a bit sad!"
But it's plain that the children do not go short of what really counts. While we talk at the kitchen table, Tansy helps Margaret to make three loaves of bread, a batch of cakes and a mushroom sauce for her siblings' supper. "It takes very little to make us happy because we love the natural environment, and that comes free," says Mark.
But what about the string of unexplained incidents and threats that has dogged their lives since Mark began his campaign? Like the local resident with several large dogs who moved into the area shortly after they arrived at a farm in Devon - and left shortly after they left. His campaign of abuse culminated in what they dubbed "Bloody Sunday", after he began firing at their milking parlour while Mark sheltered inside.
"I was pretty upset after the police said Mark would have to be shot before they could do anything," says Margaret, who was nine months pregnant at the time. Mark is well aware of the dangers of sounding paranoid, and admits that he has no proof of a campaign against him. But he points out that in the US it is well known that the chemical industry employs people to harass its opponents, so why not here?
Mark's pragmatic response has been to make sure that any incidents are recorded in the local or national press - when they suddenly seem to stop. He says that they were forced out of the Devon farm in the end. "We found a place in Wales, but the day before we were due to move in, it burned down."
"We couldn't quite believe it," says Margaret, "so we asked friends to go and have a look. You start doubting yourself."
In 1993, as the Government held an inquiry into OPs and human health, another apparent weirdo began to make their life a misery. "He claimed he was a journalist, and he was always just about to do a story," says Mark. Then he was sectioned in a mental hospital, broke out and "went on the rampage. He went to someone else's house first - another anti-OP campaigner - and broke her husband's fingers," says Margaret.
Next, he went to the Purdeys', where Mark got rid of him by telling him police were watching from a distance. Despite calls to the police, the man was on the loose for another four days, say the Purdeys, while Margaret had to take the children to a "safe house" in another village. She says they were not scared, although they knew what was going on. No doubt they took their line from her.
The campaigner who had been attacked considered laying a complaint against his assailant for GBH, says Mark, but "it's difficult to prosecute someone who has been sectioned."
There have been other unexplained events. Such as the wall of their barn falling on to the caravan which housed Mark's medical library, on one rare night in 1991 when they were away. "We had just got planning permission for the barn, which means it was sound," says Mark. Then, in 1995, Margaret was alone at home with the children when she found a man rummaging around in their outbuildings. "He freaked Margaret out by saying: 'I'm just on my way to Guernsey' - which is where I happened to be speaking that evening," says Mark.
In 1993, on the night before a story about the Purdeys by Susan Watts appeared in The Independent, their telephone lines were vandalised, making it impossible for Mark to respond to any follow-up media interest.
"All these events put a great strain on our relationship," says Mark, "and we were arguing a lot."
"We did have huge rows," agrees Margaret. "I would blame Mark - for being involved in all this in the first place." Yet she has never tried to dissuade him. "If he wants to spend his entire life helping people, it's up to him, isn't it?" she says.
What really drives her mad are the things that cause tensions in any relationship - such as Mark not being there for her and the children - but to a greater extent. Bump-starting tractors, delivering cows by Caesarean, doing the milking before the children wake up, trying to start the Rotavator: these are jobs she has had to do without Mark's help because he has been away, or writing a paper or - interminably - answering queries on the telephone.
For as soon as he comes in from his farm work at 6pm, he starts writing articles and studying - often until the small hours. Meanwhile Margaret puts five children to bed: "They say, 'When can Daddy tell us a story?' I say, 'Look, you should respect him for what he is doing.' But they want a normal father. I can see it from both sides, but it's upsetting. And these days we have no conversation except about OPs - whereas once we used to talk about poetry."
"I do feel guilt," Mark admits, "but I'm so angry at the way the Government has duped the public. It's my duty to carry on." He has been banging the table with his fist and now Tansy is imitating him, much to Margaret's amusement.
Mark is now convinced that in trials which he commissioned at the Institute of Psychiatrists, and funded with the help of well-wishers, he has gone a long way towards proving his theory about OPs and BSE. In April he will present his evidence to the Government's advisory committee on spongiform diseases (SEAC).
Margaret says that although she doesn't understand the science, Mark has been right before. "He was ridiculed at the children's school for opposing the use of head lice shampoos containing a kind of OP chemical [malathion]." Recently, however, official advice has changed, to promote the use of alternatives to these shampoos.
"We've been on an amazing journey together," says Mark. "That's quite bonding. It makes you respect and honour your partner, and we hardly row any more."
"There's no one else we could share our past with," says Margaret wryly. "No one else would believe it."
So what of the future? Mark's theory may be wrong, in which case, he says, "life goes on". Or he is right, in which case he could find himself acclaimed as a saviour.
There is a third possibility, which Margaret half-expects: that his theory is proved right - but he gets no credit for it. In which cases she could be sharing the good jumper for some time to come.Reuse content