Excavation work is now under way at a Gloucester cafe where Mary had been working during the period she went missing, after the site was linked to her disappearance earlier this month.
She is suspected to have been one of the victims of notorious serial killer Fred West, who was charged with committing at least 12 murders in the area over a 20-year period.
Along with his second wife, Rose, West tortured, raped and murdered an unknown number of women between 1967 and 1987.
To date, there are 10 confirmed victims of the pair – with Fred West having been charged with a further two killings.
A TV production company filming at the Clean Slate Gloucester cafe near the beginning of May 2021 discovered blue material buried at the site.
Mary, then aged 15, was wearing a blue jacket, as well as a blue and white dress and was carrying a blue bag when she went missing in January 1968 – at a time when Mr West was abducting girls in the area.
She was last seen on Bristol Road, Gloucester, just a few streets away from the Clean Slate cafe – then called the Pop-In – and from Fred West’s then home.
Officers arrived at the cafe at approximately 8am on Wednesday, May 19, to begin the excavation process. A senior officer said the work would be carried out "carefully and painstakingly" and that it could take at least a fortnight to complete.
Speaking on Tuesday, May 17, Detective Chief Inspector John Turner, who is leading the investigation, said West was a regular at the cafe and knew Mary since she worked there as a waitress.
West is said to have confessed to killing Mary in conversation with his son, Stephen, but has never admitted it to police.
Detectives have recently spoken to Stephen West, Mr Turner said.
Mary’s body was not found during the 1994 excavation of the Wests’ home, now known as the House of Horrors, on Cromwell Street in Gloucester.
The detective said there was conflicting evidence during the original ‘House of Horrors’ inquiry as to whether West, who worked as a builder, had carried out renovation work at the cafe in 1968 or later, in 1976.
“Since then there has been a lot of rumour and folklore about Mary and Fred’s link with the cafe and the decision was made that there was insufficient evidence at the time to excavate the site,” Mr Turner said.
“I met the Bastholm family on Monday and spoke with them at length about what we were going to do and the plan moving forward.
“Clearly since 1968 the family haven’t known what’s happened to Mary.
“There has been some speculation that she may have been linked to Fred West at that time and there was conflicting information within the investigation around that.
“But it is really important to them that we do actually now search for the truth and find out once and for all if Mary is buried under the basement.”
In a statement, Mary’s family said: “We are extremely happy Gloucestershire Police is continuing to try and search for Mary and this gives us a chance to potentially put her at rest after all these years.
“We hope this is a chance to finally get closure for Mary and would like to continue to ask for privacy whilst the excavation is ongoing.”
West took his own life in a Birmingham prison before his trial for the 12 murders he had been charged with. Rose West, meanwhile, was convicted of 10 murders in November 1995 and is serving life.