“You will never get another case like this.”

The words of Thomas Ross QC perhaps explain why a documentary crew was keen to capture the murder trial of the carers who killed Margaret Fleming.

The case unfolded evidence to prove Margaret was killed at just 19-years-old between December 1999 and January 2000 by her carers Edward Cairney, 77, and Avril Jones, 59, who then covered up the crime to claim £182,000 in benefits.

Vulnerable Margaret moved into the pair’s home in Seacroft, Inverkip in October 1995 when her father Derek died.

What unravelled is revealed more than two decades later in Murder Trial: The Disappearance of Margaret Fleming which will be aired on the BBC Scotland channel on January 7 and 8.

It features High Court of Scotland Prosecutor Iain McSporran QC and defence counsel Thomas Ross QC, who represented Edward Cairney and Ian Duguid QC, who defended Avril Jones.

The documentary team was given unprecedented access to the inner workings of a high court murder case resulting in the two-part series which delves into the pasts of both the victim and her killers who will each serve a minimum of 14 years in jail.

The documentary follows on from when the public was first introduced to the pair in a TV interview with BBC News reporter Suzanne Allan.

In the interview, which aired in October 2017, Jones and Cairney denied having anything to do with Margaret’s disappearance and even suggested she was alive and working as a gangmaster in Poland.

Mr Ross, 56, was given the difficult task of defending Cairney when he already put that evidence into the public domain.

He said: “It is one of the parts of evidence that you can’t really do anything about.

“I don’t know why he did the interview, it was there it had been recorded.

“He doesn’t need to prove that he is innocent. He doesn’t even have to prove that she is alive.

“The crown has to prove that she is dead.

“We knew it was there and would have to be dealt with.”

During the trial, another key piece of evidence came from PC Jonathan Gilmour who told the jury that Cairney predicted the case would end up in a murder charge.

Mr Ross said: “Looking at it in isolation, it sounds very significant.

“But it started with two routine police officers coming to check then another two officers then two CID then a dog unit then somebody talking about helicopters and divers.

“When you have gone from two officers turning up for a missing person to your house full of about 15 - and saying this is going to end up a murder charge, it was in the context of the police enquiry.”

Mr McSporran, 55, added: “That is a good example of why we have two sides because I made something of it in my speech I am sure.

“Why would you be talking about murder if someone was just away being a gangmaster in Poland?”

The relationship between Jones and Cairney is also explored in the documentary and no one – not even their own defence – is able to determine what went on between the pair.

Mr Ross said: “We didn’t get any insight at all into the relationship. I am only talking about the evidence.

“We know they stayed together for years and we know witnesses said they weren’t sure if they were a couple.”

He added: “He was quite clever and he took an interest in the case in a way a lot of accused don’t.”

Mr Duguid, 64, said: “It would be quite difficult to get to the bottom of the nature of their relationship because obviously issues of confidentiality remain.

“But even to try to establish what the relationship was over a number of years was quite hard to discern really over the case.”

He added: “As a personality, she was just quite reserved. She didn’t show many emotions.

“You see her when she was interviewed on the television by Suzanne Allan. That is pretty much the embodiment of her general personality.

“Some people said she was a bit detached from the process sometimes.

“She certainly gave a good indication of following all the proceedings but it is hard to find a description for her. She is quite unemotional I think.”

From the witnesses who take to the stand to the people who knew Margaret growing up, the documentary takes the viewer on a journey placing a woman who society forgot about at its very heart.

Mr Thomas said: “You will see how hard people work to prepare the case and be organised.

“There is also a misconception that lawyers can be snipey, grandstanding or theatrical. There is not much of that.

“It is obvious that the witnesses were treated decently and respectfully.”

Catch Murder Trial: The Disappearance of Margaret Fleming on BBC Scotland on January 7 and 8.