A police officer used “instinct” to snare crooks who murdered a disabled woman to steal her benefits.

Sergeant Chris McKay grew suspicious during a routine call to check on a vulnerable person.

He did not know that Margaret Fleming, who had learning difficulties, had been killed more than a decade earlier, when she was 19.

But when he paid a visit to her carers he instantly spotted something was not right.

Chris vividly remembers meeting oddballs Edward Cairney and Avril Jones at their home in 2016.

He recalled: “Our job from the outset was to find Margaret. That’s what ­response cops do on a daily basis.

“But this wasn’t an ordinary case. From the outset, it was very strange.”

Chris was struck by Cairney’s strange behaviour during the visit.

Edward Cairney's (pictured) behaviour worried Sergeant Chris McKay

He said: “Eddie would control the conversation. He wouldn’t allow Avril to speak. You could tell she was extremely nervous, just very fidgety with her hands, and looked a bag of nerves."

At the time, Margaret had not been seen for 17 years.

Cairney and Jones told people she had gone to live with Travellers and hoped that would mean no-one suspected anything was wrong.

They tried to convince Chris with more lies, but he did not believe them.

He said: “The one thing Eddie said that stuck with me was that if we went away, Margaret Fleming would come back because authority figures had it drilled into her that she would be taken into care.

Margaret Fleming's life was cruelly cut short

“That’s when we were thinking, ‘Wait a minute, something is not right here’.”

Chris then raised the alarm and a ­police investigation was launched.

Soon after, bone fragments were found in the garden at their home in Inverkip, Inverclyde. Experts said some could be human but they were so small it was impossible to tell.

In 2019 Cairney, 78, and Jones, 60, were convicted of murdering Margaret between December 1999 and January 2000. The sick pair – who faked letters from the teenager – had continued to pocket her benefits for 17 years, a total of £182,000.

Police search at home of Margaret Fleming in Inverkip, Inverclyde

They were sentenced to life in jail for murder and perverting the course of justice. Jones was also convicted of fraudulently claiming the 19-year-old’s benefits.

To this day, Margaret’s body has never been found.

The case is the focus of a documentary next month on Crime + Investigation series Murder At My Door, hosted by ­Kym Marsh.