His father Derek, a West Country sheep farmer in his forties, had been better too: he had been able to walk for up to three-quarters-of-a-mile at a time before his legs gave out. He had been breathing more easily and getting less depressed. It did not seem quite so important that the family had made a point of getting rid of the shotgun they had always kept about the house.
But then, a couple of weeks ago, the wind blew hard from the south-east carrying something upon it that was harmless to most people, but brought the pain and the other symptoms rushing back to Derek and Jim. They are just two of many hundreds of people in Britain whose lives, it seems increasingly clear, have been ruined by substances developed from Nazi nerve gases, which are routinely spread over the country and rubbed into children's scalps. They contaminate our food and water and are freely available over the shop counter.
Protest has swept the country since the Government finally admitted ten days ago that poisoning by organophosphate (OP) pesticides may have caused the Gulf War Syndrome that affects some 750 Britons, and since last week's Independent on Sunday revealed that the soldiers had to spray the chemicals without being issued with protective clothing.
But another tragedy has been simultaneously developing, almost unnoticed, at home. Evidence is steadily accumulating - though ministers are still refusing to accept it - that hundreds of British farmers and their families have developed a similar pesticide-induced syndrome.
Jim lost his childhood before he was born. Almost exactly seven years ago, before she even knew she was pregnant, Alice, his mother, had helped his father to dip their 600 sheep. It had to be done; between 1976 and 1992 the Ministry of Agriculture compelled all farmers to dip at least once a year to combat sheep scab. OP pesticides were the only ones to do the job cheaply.
The boy was born with a tumour and had to have an operation when only six days old. That was just the beginning of his troubles. The consultant who is treating his father said yesterday that Jim's health had been "shot to pieces".
"He gets terrible pains in his head and legs," says Alice, "and the ones in his stomach make him cry out in agony all night. He gets nightmares, peculiar temperature surges and becomes very breathless. His legs give out and he falls over. And he forgets things: most recently he has been unable to use a knife and fork, something he could always do.
"How can a little boy understand what is happening to him? What do you say to him when he is in pain and he asks why the medicine is not working?"
Derek's illness seems to date from around the same time, and has been steadily getting worse, though he still has to work the farm to make a living. "On a good day I can walk half- to three-quarters-of-a-mile before my legs give out," he says. "On a bad one I can barely make it to the barn.
"I get terrible mood swings, and very depressed. There are severe headaches and I get dizzy, sometimes I feel myself almost drifting away. My sense of smell has gone, I get hot flushes, and I am very short of breath."
From July until a couple of weeks ago, Jim and Derek had a brief reprieve - until it was snatched away. One of the effects of OP poisoning is that its victims can become so vulnerable that their symptoms can be set off by the merest whiff of pesticide, petrol, or a cheap perfume. The Percys believe that a faint trace of some chemical, carried on that south-east wind, was enough to plunge the family back into its nightmare.
Polly Evans tells a similar story. An author of children's books, she moved with her family to a small sheep farm on the Welsh borders just over ten years ago, for "the fresh air". But they found that pesticide spray drifted over their land from neighbouring farms and they were forced by the ministry to use OP dips for their own sheep even though, as an environmentally-conscious family, they had wanted to use an organic, rather than any artificial pesticides.
For the last eight years, she, her husband and both their sons have been suffering from the syndrome increasingly reported from all over the country. But they could not get doctors to take it seriously. "It has been a tremendous struggle. We have been feeling so desperately ill, but the doctors have been saying it is all in our minds," she says. "They would say, 'I know you are feeling these things, but you really need psychiatric help. There is nothing we can do about it'."
Finally, just last Friday, they found a GP, Dr Sarah Myhill, who understood and recognised their condition. Yesterday she said that she believed both the Gulf War Syndrome and the illness suffered by so many sheep farmers were caused by OP pesticides. She added: "It is now very much on the increase."
Such is their plight that both the families described here asked to have their identities disguised, particularly to save their children being stigmatised at school. Their names have been changed. But a few of the more than 500 people who have reported the syndrome are starting to go public.
Gary Coomber, a 37-year-old Kent sheep farmer, has suffered repeated heart attacks, among other symptoms; he is one of 28 victims (most of whom cannot be named) to have been granted legal aid in order to pursue claims for compensation.
And last week the Independent reported that Tom King, who was both Defence Secretary during the Gulf War and Environment Secretary some years earlier, had been ill after sheep-dipping on his wife's farm.
Robert Billings' story is the most bizarre of all. A well-respected 60- year-old farmer from Warninglid, West Sussex, he was convicted in January 1995 of attempted murder, and was jailed for 12 years. He had convinced himself that a farm labourer, George Foster, was having an affair with his wife-to-be, shot him and paralysed him.
He had been dipping sheep the day before the shooting. When he learned of the dangers of OP pesticides while in jail, he lodged an appeal on the grounds that they had affected his mental state. In July, Lord Bingham, the Lord Chief Justice, granted him a re-trial on the grounds that his conviction was "unsafe". After hearing psychiatric evidence, Lord Bingham had concluded that the jury at the original trial "did not have the opportunity to consider the possibility that this man's intent was affected by OP poisoning".
Dr Robert Davies, the psychiatrist who gave evidence for Mr Billings, said yesterday that "impulsive aggression" was one of the symptoms of the emerging sheep-dip syndrome. He also believes that it may be one of the reasons for the remarkably high suicide rate among farmers. Nineteen of the 22 people he has personally diagnosed as suffering from the syndrome have entertained suicide, he says.
The Countess of Mar - who has suffered from ill-health since getting sheep dip in her gumboots seven years ago, and has long campaigned in the House of Lords for a ban on OPs - has frankly told a meeting of farmers: "I have stood with a shotgun under my chin on two occasions, and ten minutes later wondered why I was doing it." And research from Almeria in Spain, where OPs are particularly heavily used, has reported suicide rates four times the national average.
Pesticides, by definition, are designed to kill. The OPs have evolved from nerve gases developed and stored by the Nazis in the Second World War. In the late 1930s, a German scientist, Dr Gerhard Schrader, discovered an OP compound - which he modestly named Schradan - and started manufacturing trials.
He soon started suffering side-effects, including "an inexplicable action causing the power of sight to be much weakened; it was hardly possible to see to reach home by car". Just a drop, spilled on the laboratory bench, was enough to cause him great difficulty in breathing.
Schradan, rechristened Tabun, became the first nerve agent and was taken over by the Wehrmacht - the German army - put into production and stockpiled. The organophosphates that have been developed since vary widely in toxicity, but they all work in the same way by acting on the nervous system. And increasingly research shows that repeated small exposures can cause damage as well as single big ones.
As reported on page one today, a top-level British government committee, chaired by Professor Solly Zuckerman, warned of the dangers of OPs in 1951 - right at the beginning of their evolution - and recommended stringent and immediate precautions in their use, which were never implemented.
Ironically they came into their own because of environmental concern about their predecessors, organochlorine pesticides, whose long-term persistent effects on wildlife were exposed in Rachel Carson's famous book, Silent Spring. The organophosphates do not last nearly as long in the environment, and so are generally better for wildlife, but they are more acutely toxic to people.
And evidence is growing that far more people than the hundreds blighted by acute sheep dip syndrome have been affected, albeit in many cases to a far lesser degree.
One study suggests that a third of British farmers have suffered some damage to their nervous system. Another report, by the official Health and Safety Executive, which compared sheep farmers with quarry workers, found that they found it much harder to process information in their brains.
Though ministers continue even to deny the existence of a sheep dip syndrome, senior Department of Health officials are now beginning to accept that low doses of OPs do indeed damage the central nervous system.
It is, however, the Gulf War that has put OPs on the political agenda. About 750 of the British troops serving in the war - and some 15,000 of the Americans - have since complained of a wide variety of symptoms including listlessness, depression, nausea and pain, which some have blamed on OP pesticides, among other possible causes.
Earlier this month - after previously dismissing the veterans' claims as "a mixture of unsubstantiated rumour and incorrect information" - Nicholas Soames, the armed forces minister, finally admitted that OPs had been used in the Gulf and could be linked with the syndrome.
OPs may even be connected with that other great environmental medical scandal of the year, BSE. Mark Purdey, a Somerset farmer who has been studying the disease for ten years, believes that it was caused by increasing use of an OP pesticide to treat cattle for warble fly and that this damages the cows' central nervous systems and could have distorted the "prion" proteins suspected of being the agent of BSE. His thesis is controversial even among OP campaigners, but after years of being dismissed as a crank he has now been asked to give his evidence to the National CJD Unit in Edinburgh.
OPs are everywhere. They are in our food: earlier this year they were found to be 25 times higher than expected in some carrots. They get into water: the National Rivers Authority found that a sheep dip chemical was the most frequent cause of breaches in standards for pesticides in water last year. And they get into homes in flea powders, insecticides, and in headlice preparations for children.
Last Friday help lines were swamped with calls after Mrs Stella Mullineux told the radio programme, You and Yours, how her four-year-old daughter Arianne had had hallucinations after having her scalp treated with Malathion, a mild OP, to kill lice. Community Hygiene Concern, a charity with funding from the Department of Health, insists that no chemicals should be used on children in this way - it believes that the OP is in fact less dangerous than the other two main pesticides, of different types, used to fight lice - but what it recommends is a system of combing the insects out.
No one is suggesting that the everyday use of pesticides around the home is going to lead to children having the same terrifying symptoms as Jim Percy. But some doctors are arguing that no levels of the pesticides are safe, and that children are particularly vulnerable.
They fear that their use could severely damage immune systems and lead to aggression and under-achievement at school. Dr Myhill last week described all headlice treatment as "a disaster" and Dr Davies says that anything that is used directly on children's skins "should be taken off the market".