Sandy’s parents Margaret and Phillip believed that their son had been taken by a lonely man who wanted a child of his own.
That theory gained credence over a decade ago when a workman from the building site contacted Donna after she made an appeal in a local newspaper.
Ms Davidson said in 2017: “He said he saw Sandy going away and holding a gentleman’s hand but he didn’t feel any cause for concern because he wasn’t struggling and seemed quite happy.
“It has played in his mind all this time because he was probably the last person to see him alive. It has affected so many people.”
The primary school being built at the time of the disappearance was demolished in 2014, but despite Donna and her family’s pleas the land was not fully excavated.
Occasionally the family have been given hope, only for it to be quashed. In November 2013, someone born around the same time as Sandy came forward and underwent a DNA test. However it only proved he was not Sandy.
Two years later, someone contacted Donna through a missing persons support Facebook group, alleging they were abducted by a teenage girl around the time of the disappearance. Police were not able to link the incidents.