Accused. Tried. Convicted. Imprisoned. Freed. Accused. Tried. Acquitted. Angela and Ian Gay know the inside of the British criminal justice system better than most. Their four-year agony stopped on Friday when a jury in Nottingham found them not guilty of poisoning with salt Christian Blewitt, the blond-haired little boy they were in the middle of adopting.

Five million quid down the drain, a trial, an appeal and a second trial later, we all know that Angela and Ian are innocent. They didn’t torture their little boy by force-feeding him with 4 teaspoons of salt. Instead, the jury opted for natural causes — accepting the defence argument that there was something wrong with Christian. Specifically, that the bit of the brain that regulates the salt-water balance in the body, the osmostat, was faulty. Christian was like a diabetic who suffers a coma and dies, only it wasn’t his blood sugar levels but his salt-water balance that went haywire.

Since his death in 2002, the police, the Crown Prosecution Service (CPS) and then the prisoners that tormented the couple as child killers when they were banged up have all made a false assumption that it was deliberate salt-poisoning.

I tested that assumption myself by drinking a pint of water with 4 teaspoons of salt in it — and threw up immediately. Because I was making a film for Newsnight, I had to do it five times, mid-shot, close-up, long shot, and each time I vomited. Salt’s an emetic. At no time has anyone ever explained how the Gays got nature’s No 1 sick-making substance down the throat of a three-year-old.

Now it is the turn of the system that twice accused the Gays to account for its actions. The defences are already in: the police did their duty to investigate allegations of child abuse; the CPS say they were right to prosecute, twice.

There is strong evidence that the system is a serial offender, guilty of bringing false accusations of child killing, that it does not ask itself simple, common-sense questions, such as: “Was there a crime?” The system is so driven by a terror of letting another Victoria Climbié tragedy happen that it repeatedly prosecutes innocents on junk evidence.

The system’s first mistake is its failure to learn. We’ve been here before: Sally Clark, Angela Cannings and Donna Anthony were all falsely accused of child murder on the basis of flawed evidence. The man who gave evidence in those disastrous trials, Professor Sir Roy Meadow, wrote the seminal paper on salt-poisoning. Meadow did not give evidence in the trials of the Gays, but he laid the intellectual groundwork for the prosecutions. His paper on salt-poisoning, like his work on cot deaths, is dross. Jean Golding, professor of epidemiology at Bristol University, has compared Meadow’s scientific method to “stamp-collecting”.

The system’s second mistake is its failure to ask whether a crime took place. The attorney-general should spare time to read the notes of the original meeting of the experts, chaired by the crown’s junior barrister, Andrew Lockhart, before the first trial of the Gays. Nowhere does anyone ask, “are we absolutely certain that we have properly ruled out natural causes?” Instead, foul play was assumed.

Most criminal trials turn on the question: “Who did it?” When a baby or a young child dies, the same logic applies. But that’s crazy. Common sense, backed by government statistics, tells you that the great majority of infant deaths happen by natural causes. The question “who did it?” launches a hunt for perpetrators and knocks flat the primary question, “was an ‘it’ done?”

The system’s third mistake is to overvalue expert witnesses as demigods and write off friends and family as irrelevant. At the first trial, the crown’s lead expert on salt-poisoning was Professor George Haycock of Guy’s hospital. He did the calculations that said the Gays must have poisoned Christian with 4 teaspoons of salt. The Gays were convicted.

Enter Dr Glyn Walters, a retired chemical pathologist. He is an oldie with two hearing aids, but cross him at your peril. His calculations said: natural causes, a faulty or reset osmostat. Haycock told the Court of Appeal that he couldn’t knock down Walters’s numbers. The Gays were freed — but then told by the court that they had to face a retrial.

At no time did the crown ever put any weight on the good character of the Gays and their families. I got to know all of them a little after they were freed from prison. Their families are all patently good people. Ian and Angela are gentle and caring. She lost her womb to cancer when she was 16. That’s why they wanted to adopt.

How could it be that the expert Professor Haycock was relied upon so heavily at the first trial but dropped by the prosecution for the second? Meanwhile, the views of a score of the Gays’ family and friends were discounted at both trials. Could it be that one is a middle-class professor and the rest people with Black Country accents?

When a child dies of abuse, and the authorities have been judged to have bungled, all hell is let loose. A public inquiry often follows. But when the opposite wrong happens — a child dies of natural causes and their parents (natural or adopted) are falsely accused — nothing much takes place. So long as the system remains convinced that it hasn’t done anything wrong, we can be pretty confident that someone else will end up in prison for a crime that never happened.

BBC reporter John Sweeney has helped free seven people from prison.

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