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While the circumstances are very different, Norris' supporters also argue that his case hinges on disputed interpretations of medical evidence.

The story begins in 2002 when Norris, a 26-year-old nurse, originally from Glasgow, found Ethel in an hypoglycemic coma as she recovered from a broken hip.

Doctors took blood samples to find out what caused her blood sugar to drop so low.

The test results seemed conclusive - she had a massive amount of insulin in her blood.

She died 21 days later, sparking a murder investigation.

A nurse who had been on shift on the night Ethel was found told the police that Norris had predicted the 86-year-old might not last the night.

He quickly became the prime suspect.

The police operation trawled retrospectively through all the deaths on Norris's wards, looking for potential murders that may have gone unnoticed.

They identified three other cases of unexplained severe hypoglycemic deaths and one where the patient recovered.

No blood tests had been taken from these cases at the time of their deaths to be able to prove insulin poisoning but experts for the prosecution said in court that their medical charts showed persistent and severe low blood sugar readings.

They said that because this was so rare in non-diabetics that it must be murder.

When it came to court in 2007, experts called by the defence said four of the five patients had died of natural causes.

But after a five-month trial and a deadlocked jury, Norris was convicted by a majority of 11-1 and told he would not be eligible for parole for 30 years.

After the verdict, Ethel's son Stuart Hall said: "I don't think he should come out again. He's got the knowledge to commit murder but do it discreetly."

When I got involved in the case in 2009, Norris, who is now known as Colin Campbell, had just lost his first appeal against conviction.

Two years later, new scientific evidence came to light which challenged one of the key assumptions underpinning the prosecution case.

It claimed hypoglycemia - low blood sugar - in the frail and elderly is not as vanishingly rare as was once thought, suggesting the jury had been misinformed.

This new evidence was submitted for a second appeal, which took another 14 years to be heard.

During that appeal last year, experts for Norris argued that the symptoms seen in several of the patients could occur naturally in elderly, frail people.

Experts for the Crown maintained the deaths were medically distinct.

They said their blood sugar readings were refractory, meaning they kept coming back despite being given glucose, whereas naturally-occurring hypoglycemia can be easily corrected with a treatment such as a sugary drink.

They argued this distinction could only be explained by deliberate insulin poisoning.

Were the syringes accidentally switched?

Initially, we discounted this scenario because we assumed medical staff would be alerted if a diabetic patient had accidentally been given dextrose instead of insulin.

However, endocrinologist Prof Joel Zonszein, an expert in diabetes, said the amount given would not have caused a severe illness.

"The answer is very different from somebody getting insulin," he said.

"Insulin is much more aggressive. Giving too much sugar does not cause a coma and it will take more that 24 hours until the person goes into a coma. It's completely different."

Prof Zonszein also said the amount of insulin Ethel may have been given accidentally would have produced the high readings in her immunoassay blood test taken five hours later.

He said the drip of dextrose would have masked the insulin, making it harder to notice, but the blood test result would still register a very high level of insulin.

If the syringes were accidentally switched, is that a mix-up rather than a murder?

That still leaves another question: what caused Ethel to fall into a coma in the first place?

Our investigation found all five patients had been prescribed medications, all were on antibiotics and two of the women were on powerful painkillers.

All of these are known to increase the risk of severe hypoglycaemia in frail, elderly patients.

It is possible that a strong opioid prescribed to Ethel for her post-op pain put her in a coma, and insulin meant for somebody else was accidentally given to her before the blood sample, which proved poisoning, was taken.

The Court of Appeal ruled last year on whether naturally-occurring hypoglycaemia could explain the medical collapse of the four patients beyond Ethel Hall but ruled the case put by Norris' team got nowhere close to undermining the safety of his convictions and rejected the appeal on all grounds.

They also rejected Norris' lawyers' attempts to go to the Supreme Court.

An application has been made to the European Court of Human Rights, by Norris' lawyers, but solicitor Rory Hearty says this "hurdle" will take many years.