They will spend a week at the Hertfordshire farm where Mrs McKay, the wife of a newspaper executive, was held for a £1m ransom before her kidnappers were arrested and convicted of her murder.

Detectives have mapped out a small area to excavate and have assembled a team of specialist searchers, forensics experts, archaeologists, and photographers.

Part of a barn will be pulled down as it was later erected over the edge of the land to be dug up.

Muriel's family has campaigned for a new search for two years and had to persuade the Metropolitan Police and the farm's owner to approve it.

Police excavated a nearby area in 2022 and found nothing, but the family said they dug in the wrong place.

Muriel's son Ian, 82, has flown from his home in Australia to be with family members and to visit the excavation site.

"This is a highly emotional time for all of us and we wanted to be together," he said. "My sister Dianne and her son Mark Dyer have done a wonderful job gathering evidence and passing it to the police.

"This is pretty earth-shattering stuff if we are able to find her after all this time. I remember the kidnapping vividly, of course, but I didn't really speak about it later to my wife and children. It wasn't something to chat about."

Muriel was never seen again after being snatched from her London home by brothers Nizamodeen and Arthur Hosein just before New Year in 1969.

They mistook her for Anna, the wife of media mogul Rupert Murdoch. Muriel's husband was Alick McKay, Murdoch's deputy in the company that had just bought the Sun newspaper.

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In January this year, Muriel's daughter Dianne McKay and her son flew to the Caribbean when they discovered Nizam had been deported to his native Trinidad after serving 20 years for the murder.

He and his brother Arthur - who died in prison - had always denied involvement in the crime, but after meeting the family Nizam told them Muriel had died of a heart attack within days of her kidnap and, in a panic, the brothers buried her behind a barn at the back of the farmhouse.

Using old maps and photographs to identify the burial site, Hosein, 76, told them in a meeting filmed by Sky News: "I came out of the farmhouse, through the gate and turned left. Three feet from the fence, that's where the body is."

Muriel's grandson Mr Dyer said: "We've been waiting for this for a long time. I hope by the end of the week we will know if we can bring my grandmother home and give her a proper burial. This is really our last chance, but we are hopeful."

For the duration of the dig, police have imposed an air exclusion zone and guards will man the entrance and footpaths leading to the farm in Stocking Pelham near Bishop's Stortford.

The McKay family has said that if police do not find Muriel's remains, they will not press for another dig at the farm.