In his prison cell, Callan pored over medical and legal textbooks and corresponded with internationally renowned neurologists to prove the expert testimony given at his trial was flawed.
In April, almost exactly four years after the death of the child he had loved and cared for, he was finally freed by the Appeal Court.
Callan, now 37, remembered his feeling of helplessness during his trial: "I heard the judge telling the jury that the pathologist had been doing his job for 30 years and the paediatrician for 25 years - 55 years of experience between them. Why would the jury believe me against all that?
"I didn't know what to do. I thought the guys defending me must know what they were doing. I heard the pathologist using all these technical terms for parts of the brain and I thought, he must know what he's talking about - how could I challenge them?"
But, driven by his obsession to prove his innocence, he did.
Now reunited with Mandy's mother, Lesley Bridgewood, and struggling to survive in a caravan in north Wales as he waits for compensation for his wrongful conviction, he is angry that the legal system did nothing to ensure that the right experts were involved in his case.
"They should be experts within their own competence and not go outside that," he says. "You wouldn't expect a neurosurgeon to give evidence on the body, so why ask a forensic pathologist about the brain? The court system in this country is not about a search for the truth. It is like a game, but it put our two families through hell."
His QC, Michael Mansfield, says Callan's case illustrates the need for an urgent review of the use of expert witnesses. "The problem is science is highly subjective but it is regarded as though it is written in tablets of stone. Over the past 10 years, eight out of 10 miscarriages of justice have involved an element of unreliable scientific evidence."
It was concern over the standard of expert witnesses that prompted barrister Michael Cohen and 10 others to set up the Academy of Experts eight years ago.
It now has 1,500 accredited experts on its register covering everything from arboriculture through meterology to zoology.
Everyone has been vetted, not just for technical competence, but also to ensure the applicant has a full understanding of what it means to be an expert witness in a contentious case.
Academy chairman Cohen, who also acts as an expert witness in insurance issues and as an arbitrator in boat and other commercial disputes, says experts should not act as hired guns going into battle for the side paying them.
"It is not up to them to dream up wonderful scenarios about what might have happened and then to defend those theories to the court. Experts are required to play two distinct roles. At the beginning, they are part of the team putting together the case, advising on possible lines of inquiry.
"Then they are appointed as the expert witness and instructed to prepare a report. From then on, they must be independent. One side may be paying them but their duty is to the court.
"One of the biggest problems is that you can find somebody to say anything."
For Cohen, cases such as Callan's and other miscarriages keep bringing him back to one question: "If incompetent technical evidence is given during a trial, what were the lawyers doing?"
The Academy's expert search service receives up to 1,000 requests a year, primarily from lawyers, but also from foreign governments and companies needing specialist advice.
For example, one expert in constant demand in disputes over workmanship is a stonemason. "There isn't a national accrediting body for stonemasons," Cohen says, "so it took us 14 months of investigations into his technical skills and professional standing before the vetting committee was satisfied he was up to scratch.
"We recently turned down an exorcist and a water diviner. We have also had a spate of calls for an expert in demonic possession, but don't profess to have one on our register. Our criterion is basically any discipline that would be accepted by a court."
The Academy, which holds training courses on the responsibilities of expert witnesses, ethics, report-writing and courtroom skills, costs about pounds 200,000 a year to run. It is largely self-supporting through the pounds 120 membership fees, its search service and courses.
The Academy has also drawn up guidelines for forensic scientists to follow when acting as expert witnesses. "The judge, jury and lawyers can't check all the technical details. But if they can see the expert has stuck to an agreed protocol in the way he carried out the tests, they can get a feel for the weight they can put on his evidence."
The Academy's judicial committee, which includes senior judges from the House of Lords and the three appeal courts in England and Wales, Scotland and Northern Ireland, has banned its experts from being paid on a "no win, no fee" basis.
"Expert witnesses are the only people allowed to give their opinion in court," Cohen says. "Can you imagine the pressure if you know your fee depended on the opinion you gave?"
ln civil cases, the recent Woolf report, Access to Justice, said engaging experts was a "source of excessive expense, delay and, in some cases, increased complexity" through their "excessive or inappropriate" use. Lord Woolf recommended subjecting expert evidence to the complete control of the court, which would have the power to appoint a single, neutral expert where appropriate.
Cohen, who says Woolf's recommendations reflect the Academy's principles, says: "I like to think that with our accrediting process and code of practice our members wouldn't commit heinous crimes, but every organisation seems to have a rogue, and sooner or later we will have ours.
"However, our members know that if they do breach the code of practice, they will face suspension or expulsion."
Callan, who is writing a book about his experience, backs their stringent policy. "I had to answer to the highest court in the land, but the other people involved haven't had to answer to anyone."