A team of murder squad detectives met the landowner this week at the site where one of her abductors has suddenly confessed to burying her body.

The officers are exploring the possibility of scanning a small area beneath a dung heap behind a barn.

Read more: Muriel McKay - the woman who vanished: Kidnap, mistaken identity, and a 50-year mystery

The family had already employed a ground-penetrating radar specialist who scanned the site from neighbouring land a few metres away and said the results showed ground disturbance up to four feet deep.

Until now the landowner banker Ian Marsh has rejected the family's request to scan from much closer, but said he would co-operate if the police asked him.

The move follows a sympathetic letter to Muriel's family from Scotland Yard's commissioner Dame Cressida Dick.

In response to a plea for help from Muriel's daughter, Dianne McKay, the head of the Metropolitan Police wrote: "Please may I begin by offering my profound sympathies for the pain that your family still experience.

"I cannot imagine the anguish you must feel to have lost your mother in these circumstances. This is made worse, of course, that all these years you have not been able to properly say goodbye."

"I want to reassure you that my investigation team are exploring every opportunity available to us.

"I know that a member of my team has recently met with you and undertaken to keep you updated. I have also asked him to update me as he progresses to the next stage."

And in a hand-written personal sign-off, Dame Cressida wrote: "Yours sincerely and with every good wish."

Dianne McKay, 81, said: "This was a big and welcome surprise from the Commissioner, who I know is under a lot of pressure on other things at the moment.

"It seems she is very sympathetic to our needs and if she is taking such a personal interest it must mean that the police investigation is moving forward. I'm very grateful for her letter."

Mrs McKay was kidnapped from her Wimbledon home in December 1969 by two brothers who mistook her for the then wife of media tycoon Rupert Murdoch. Muriel was the wife of Murdoch's deputy, Alick McKay.

Arthur and Nizamodeen Hosein held her at their rundown farm in Stocking Pelham, Hertfordshire, while they contacted her family and demanded a £1m ransom.

After a bungled police operation, the brothers were arrested and convicted of Muriel's murder, but her body was never found and her captors would not reveal what had happened to her.

But last year the surviving brother, Nizamodeen, 30 years after his prison release, finally told the family's lawyer that Muriel had died of a heart attack and that he had buried her close to the farmhouse.

Mr Marsh, the current owner of the farm, had confronted Dianne and her lawyer as they walked a footpath through the farm two weeks ago and told them to leave.

In a statement, his spokeswoman told Sky News: "Mr Marsh is in regular conversation with the Metropolitan Police over this matter and is fully cooperating with their investigation.

"He very much sympathises with the family and hope they can find closure over this tragedy. He now asks for privacy while the police complete their investigation."

Dianne McKay said: "We just want to find my mother and say goodbye to her and we really believe she is where Nizamodeen has indicated. It's her birthday on 4 February and it would be lovely if the police can move swiftly to recover her remains."