A man and a woman have been convicted of murdering a vulnerable woman they should have been caring for almost two decades ago.

Margaret Fleming, who had learning difficulties, vanished “from the face of the Earth” around December 1999. Her body has never been found.

Following a seven-week trial at the high court in Glasgow, Edward Cairney, 77, and Avril Jones, 59, were found guilty of murdering the missing woman.

Jurors found the couple murdered her by unknown means between 18 December 1999 and 5 January 2000 at their home in Inverkip, Inverclyde, or elsewhere in Scotland, and then tried to cover up the crime for almost 18 years.

Jones was also convicted unanimously of fraudulently claiming £182,000 in benefits by pretending Fleming, who would now have been in her late 30s, was alive.

The pair will be sentenced on 17 July.

Police launched an investigation after it became apparent in October 2016 that Fleming was missing. Routine social services inquiries were said to have sparked concerns over her whereabouts.

The case attracted major police resources and significant media attention as it was claimed the last independent sighting of her had been at a family event on 17 December 1999.

As the inquiry progressed, it appeared something “sinister” had happened and that she may have come to harm. Specialist teams combed the cottage where she had lived and excavated its grounds in their search for clues that might to help track her down.

Her supposed carers were arrested in October 2017. During their trial, which began in April, Fleming was described by prosecutors as a “friendless and lonely” young woman who had significant difficulties.

She went to live at the pair’s Seacroft home as a teenager after the death of her father because those closest to her “didn’t want her”. By October 1999, various benefits for Fleming flowed into the household, which was said to have had financial difficulties.

The crown suggested it had been tempting for the couple to have the money but not the “inconvenience” of looking after her. How and exactly when Fleming died may never be clearly known. It remains, as the defence highlighted, a case without a body or crime scene.

Holding them jointly responsibility for the death, the crown claimed the couple “literally got away with murder for 16 years”. Money was the motive for them to cook up an “elaborate scheme” to conceal her disappearance, the court heard.

They were brought down by “greed, arrogance and lies” after Jones made claims of Fleming having “fantastical” illnesses and conditions in correspondence with benefits officials.

As police closed in on the couple, they struggled to reconcile claims she both worked as a gangmaster and was capable of travelling overseas, and that she had major difficulties that entitled her to a number of benefits.

Det Supt Paul Livingstone, the senior investigating officer, said: “Margaret was a very vulnerable young woman who was manipulated, abused, neglected and ultimately murdered by the two people who should have been looking after her.

“It is clear that one of Cairney and Jones’ motivations was money … For many years, Cairney and Jones kept up the pretence that Margaret was still alive, going as far as to write letters claiming to be from her.

“Margaret was described as being a funny, caring young woman who, despite having some mild learning difficulties, just wanted to be liked and to have friends. The treatment which she was subjected to can only be described as horrific and the conditions in which she lived in were utterly disgusting and uninhabitable..”