Although Sharon Dunlop is the resuscitation training officer for Wexham Park Hospital in Slough, she panicked. "I got some Piriton - an antihistamine - and gave it to him. By then, his breathing was noisy and laboured." She ran outside as her husband, Duncan, arrived home, and they drove Benjamin to their doctor's surgery. Here his condition gradually improved, and he was allowed home when his symptoms had subsided.
Children who get peanut allergy tend to come from families with a tendency to allergy - "atopic" families. Ben's mother, Sharon, gets allergic asthma from contact with animals, especially dogs, and her brother suffers from eczema. Sharon Dunlop confesses that she had "a bit of a fetish" for Snickers bars, which contain peanuts, when she was pregnant with Benjamin. So is that why Ben is allergic to peanuts?
Earlier this year, the Chief Medical Officer suggested that mothers with a tendency to allergy "may wish" to avoid eating peanuts during pregnancy. His advice was based on a report from an expert Government committee, and followed a rise in the incidence of peanut allergy, which usually appears in babies and toddlers and now affects one in 200 four-year-olds. Other allergies are increasing, but peanut allergy is serious because it can cause fatal anaphylactic shock.
But clear evidence for the Government's advice is lacking, and the emphasis on what mothers "may wish" to do in pregnancy underlines that fact. Deborah Fox and Gideon Lack, doctors at the Paediatric Allergy, Asthma and Immunology Clinic at St Mary's Hospital in London, believe avoiding peanuts may even increase the prevalence of peanut allergy.
In a letter to The Lancet, they said that the Government guidelines that suggest atopic pregnant and breast-feeding mothers should avoid peanuts are not based on evidence, and have caused distress to mothers with peanut- allergic children.
They described seeing mothers of peanut-allergic patients who fear that their child's allergy was caused by the nuts they ate in pregnancy, and other mothers who had deliberately avoided eating them - but still had peanut-allergic children. In the case of identical twins seen in their clinic, one twin has often been found to be sensitive to peanuts while the other has had no reaction at all.
Given that identical twins have the same genetic make-up and are exposed during pregnancy to the same substances from the mother's blood, this suggests that something more than eating nuts in pregnancy causes the allergic reaction.
They do, however, accept that excessive peanut consumption could be associated with peanut allergy. Dr Fox told The Independent of one mother of two peanut-allergic children who ate peanuts, peanut butter or Snickers bars 25 to 30 times a week.
"But there is also evidence that complete abstinence is associated with allergy," she added. "When mothers ask us what they should do in their next pregnancy and breast-feeding, we advise them to eat small quantities."
Dr Richard Pumphrey, consultant immunologist at St Mary's Hospital, Manchester, is also sceptical about the Government's advice. "We see more and more young patients with nut allergy, and mothers often ask if it could be because they ate nuts when pregnant," he says. "There may be a small chance of sensitisation if the mother eats nuts during pregnancy, but there is no firm evidence that sensitisation occurs in the womb."
John Warner, professor of child health at Southampton University, and a member of the Government's working group on peanut allergy, said: "On the balance of evidence, and given that peanuts are not an essential part of anyone's diet, it seems reasonable to recommend that during pregnancy and breast-feeding, women with a family history of allergies do not have these products."
However, when Professor Warner was interviewed for an article that appeared in the British Medical Journal, he was quoted as saying: "Whether avoiding the allergen makes a difference is an act of faith."
Pamela Ewan, the consultant at the allergy and clinical immunology department at Cambridge's Addenbrooke's Hospital, says that there is "no evidence of sensitisation to peanut allergen in utero", and believes that more research needs to be done so that "public health measures can be soundly based".
Despite the lack of evidence, Dr Jonathan Hourihane, from the Institute of Child Health and Great Ormond Street Hospital, says that when mothers ask, he advises them to avoid peanuts because "no one comes to any harm from not eating them".
He agrees with the Government's advice, but says: "I don't believe in the nanny state, and anyway, a lot of mothers have already decided what they are going to do in their next pregnancy."
And when it comes to the question of giving young children peanuts or peanut butter, Dr Hourihane adds: "In south-east Asia and some parts of Africa, it's a weaning food."
Dr Fox makes a similar point in her letter to The Lancet: "In south-east Asian societies, in which groundnut is part of the staple diet, there is a low prevalence of peanut allergy."
Peanut allergy ranges from a mild reaction, with a fairly runny nose or slight skin blotches, to the life-threatening, with swelling of the throat and allergic shock (anaphylaxis). Symptoms are most severe in teenagers and young adults, and there are at least five deaths a year. These can be prevented if the allergy is recognised by an allergy specialist, and people at risk carry an automatic adrenaline syringe called an Epipen.
However, people with allergies have very little chance of getting good specialist advice: there are only 68 hospitals in Britain with an allergy clinic, and only 13 of those are run by a consultant allergist who is trained in dealing with all types of allergy. Only four of the 13 are outside London - in Cambridge, Leicester, Liverpool and Southampton.
Even in Cambridge, a centre of excellence, Dr Ewan says that another consultant is needed. The clinics are regularly overbooked, they see 2,200 outpatients a year, and the waiting-list is 15 months long, though cases that are urgent are seen more quickly.
In 1993, Sarah Reading died, aged 17, in a restaurant in Guildford after eating a dessert that contained traces of peanut. She didn't carry an Epipen, which might have saved her life, because neither she nor her family knew they existed. Her father, David, responded by starting the Anaphylaxis Campaign, which has 5,000 members. It has joined hands with the British Allergy Foundation and the Royal College of Physicians to press the Department of Health to recognise allergy as a speciality, to train more doctors in it, and also to increase the number of clinics.
After his acute episode, Benjamin, who is now two-and-a-half, was referred to Dr Jan Reiser at Lister Hospital, Stevenage, a paediatrician with an interest in allergy. Because his allergic response was so severe, Dr Reiser tested him for other nut allergies. He doesn't have them, and he can eat other, tree-grown nuts.
The Dunlops keep an Epipen in the house in case Benjamin needs it, and there is also one at his nursery. If Mrs Dunlop has another child, will she eat peanuts during the pregnancy? "Definitely not, and my advice is don't try a child on any form of nuts until they are at least three."
Anaphylaxis Campaign, 2 Clockhouse Road, Farnborough GU14 7QY, tel 01252 542 029. For a free information pack, send an A4, self-addressed envelope with two first-class stamps