The couple told their story not, unusually, because they wanted to sue somebody for the ordeal that had befallen them (the Maudes had asked for medical help several times during the previous three days). They wanted to set up a safety net to spare children the pain and danger faced by their daughter, now a happy, healthy five-year-old. They envisaged a nationwide computer system that would allow any doctor to tap a list of symptoms into a database and get back a list of possible diagnoses. “There is so much medical information available on the Internet and it seems crazy that nothing has been developed for hospitals,” Jason told me at the time.

I wondered whether their good intentions might evaporate as Isabel settled back into normal life. Instead, they have taken the first step towards realising their vision. Together with doctors, a software company and generous donations from the City, the Maudes have developed a software programme that will be the safety net that Isabel never had. The software, called Isabel and in the advanced stages of testing, is available free over the Internet to any bona fide health worker (there is no public access). It will be launched at a meeting of the Royal College of Paediatrics and Child Health tomorrow by Dr Joseph Britto, a consultant in paediatric intensive care at St Mary’s, who has been closely involved. Jason cannot believe they are so close to achieving their aim: “It still seems quite fantastical to me.”

The way the NHS is organised, Britto says, makes misdiagnoses very possible. “It’s the inverted pyramid in the NHS,” he says. “Potentially critically ill patients such as Isabel are seen by the most junior doctors in A&E. In her case, they spotted the chickenpox, then moved on. Despite the high fever and bluish discoloration — the clear features of the flesh-eating bug necrotising fasciitis — she was sent home. Two days later she had multiple-system failure and needed aggressive resuscitation. It is amazing she has come through this neurologically intact.

“During those long nights I spent in the corridor with Jason, he kept saying that surely there’s a way of arming junior doctors with more knowledge. It looked to me like something we could work on.” He shared many sleepless nights with the Maudes as Isabel recovered, and offered to work closely with them on the project. The three have become firm friends — the Maudes named their third child Joseph after Britto, who is his godfather. Isabel also has a younger brother, Sam.

Tap a list of symptoms into Isabel and it will respond with a list of plausible and relevant diagnoses of childhood conditions. Where those on the frontline — often junior doctors pressed for time — generally look for the usual, Isabel also catches the unusual. Not only that, but it will give detailed descriptions and photographs. This saves fumbling in medical files for guidelines on, for example, what to do if a child has an epileptic fit, and gives guidance if a doctor has never seen a particular condition before. The software also provides an instant link to the British National Formulary, the standard reference on medicines. In addition, medics can post details of their own mistakes or experience, anonymously if they prefer.

The software is not completely refined — the accuracy of diagnosis is running at between 80-90 per cent — but tomorrow’s presentation to the paediatric community should encourage other doctors to try it. The software is on trial at four teaching hospitals, including St Mary’s and Addenbrookes in Cambridge. These trials are ensuring that the programme is returning sensible diagnoses.

Poignantly, typing in Isabel’s symptoms turns up the suspect bacterium thought to be responsible for necrotising fasciitis, which ate away at her abdomen and pelvis. Isabel was saved by surgeons cutting away the dead flesh; she will need six further operations over the next two years to rebuild her abdomen and groin as she matures.

She has blossomed into a charming, carefree child, seemingly unaware of the trauma she faced or the project she inspired. When I visited the Maudes at their London home, Isabel trotted downstairs clutching Pee-Pee, a little cloth Peter Rabbit. The last time I saw Pee-Pee, it was hanging from a ventilator in the intensive-care unit at St Mary’s. Isabel bears well-hidden scars on her leg where skin has been removed to graft on to her tummy, but they are nowhere near as prominent as the normal grazes and bruises that are the battle wounds of most toddlers.

“Isabel loves dressing up, especially as a nurse,” says Charlotte, a former public relations executive. “She now wants to be a nurse when she grows up. We keep gently suggesting ‘doctor’ but she has firmly told us that doctors are men and nurses are women.”

Sometimes, Charlotte says, she feels as if Isabel’s brush with death happened in a remote, parallel life. “Every so often I look back and think ‘Did we really go through all that?’ We each recall specific incidents, like waking up at 4am and being told pretty much that we’d lost her. The doctors said they hadn’t felt a pulse for 45 minutes. We were told about possible brain damage. We had to wait two weeks before we knew.”

Jason adds: “If I’m talking about it with someone, tears come to my eyes. It’s a reflex inside you — you become very emotional.” The couple find it almost impossible to put into words their gratitude that Isabel survived. In some ways the experience encouraged them to have Joseph. “When you go though something like this you realise how fragile life is,” says Jason.

As head of global equity research at AXA, Jason used his City connections to secure £90,000 in donations to set up the Isabel Medical Charity. Autonomy Corporation has allowed the charity to use its software for nothing (it would normally cost about £300,000, and its head, Mike Lynch, is an Isabel trustee). Harcourt Health Services, the medical publisher, gave permission for its paediatric textbooks to be incorporated into the program. The Maudes put in £34,000 that they received when the RAC club was demutualised. There is a 30-strong editorial board of mainly paediatricians overseeing the content of Isabel. The board features representatives from the General Medical Council. All doctors and trustees are giving their services free; the site has none of the advertising that litters other medical databases.

The medical community’s response to Isabel has been “incredible, unbelievable”, Britto says. He thinks such software should be used widely throughout the health service and notes that 11 per cent of patients suffer an “adverse event” while in hospital, such as an incorrect dose of medicine or a misdiagnosis. It is no coincidence, he says, that litigation is growing. Having an intelligent computer program to refine or reinforce diagnoses might well become more important to the NHS to avoid charges of negligence. Putting computers in hospitals, where they could save lives, would also be an ideal and practical way of realising the Government’s commitment to online Britain.

Britto points out: “These errors cost the NHS a billion pounds a year. We really need structures and systems to minimise them. I have to be honest — when I have presented Isabel’s clinical features to senior paediatricians, not all of them have thought of the more uncommon complications of chickenpox.”

More dramatically, the Isabel system has already proved its worth. Britto says: “We had an infant referred to us last week with swelling in the neck and difficulty breathing. The child had surgical emphysema, where air leaks out to below the skin.

“Afterwards, we found out that somebody, possibly the father, had inserted his finger in the back of the child’s throat and caused a tear in the larynx. This is a very unusual form of child abuse. Most of us would have been hard-pressed to come up with this diagnosis.

“We used Isabel, and it came up with child abuse as one of several possibilities. Had this been available at the bedside, it would have had a huge impact, not just on that child, but other kids from that home. That’s what makes this project so gratifying — it can save so many children’s lives.”