They are asking a judge to order the reluctant landowner to allow a new search for Muriel's body.

They've also urged a coroner to help them exercise what they say is a common law right to collect the remains of a dead relative.

Scotland Yard detectives have searched part of the farm already, but the family insist they dug in the wrong place and now have a precise location.

Banker Ian de Burgh Marsh, who owns the farm, has refused to let the McKays do their own search and last week turned down their offer of £40,000 to repair any damage to his land.

He insists he will deal only with the police, but detectives working with the family are reluctant to ask a court for a search warrant for the farm in the village of Stocking Pelham, Hertfordshire.

In a formal 'letter before action', the McKays' legal team has written to Mr Marsh to advise him they will ask a High Court judge to compel him to let them dig.

The letter says: "It is entirely unjust for our client to be in possession of new information and be denied the opportunity to conduct a further, smaller search of the land with this new information.

"The legal basis of our client's claim will be its common law rights to have possession of the remains of a deceased family member."

It warns the landowner: "Ignoring this letter may lead to our client instructing us to commence proceedings against you personally and may increase your liability for costs."

The farm became the focus of the family's renewed search for Muriel McKay two years ago when her killer Nizam Hosein - freed from jail long ago and deported to his native Trinidad - broke his silence and told them he had buried her behind the farmhouse.

Killer's search offer blocked

Muriel, 55, was kidnapped from her London home at Christmas in 1969 and kept prisoner at the farm while demands were made for a £1m ransom.

She was the wife of newspaper executive Alick McKay, the deputy to Rupert Murdoch - who had just bought The Sun. Muriel was mistaken for Murdoch's then wife Anna.

Nizam and his older brother Arthur were convicted of Muriel's kidnap and murder, even though her body was never found, and went to jail professing their innocence. Arthur died in prison.

Hosein, now 75, has since claimed Muriel died after collapsing with a heart attack and in a panic he and his brother buried her beside a fence and near a barn.

Sky News can also reveal the Home Office has rejected Hosein's request to have his deportation order lifted so he could return to the farm and point out the burial site.

Muriel's grandson Mark Dyer, a businessman, said: "The Home Office decision is disappointing, but not unexpected.

"It would have been so helpful to have Nizam at the farm, because he is the only person alive who knows where my grandmother is.

"The decision not to lift the order was made by the last Home Secretary Suella Braverman, so maybe her successor James Cleverly will think differently and let him come back to help us.

"But if not and we are allowed to dig again we will have Nizam there on a live link from Trinidad so he can direct the search."

To add weight to their demands for 'a last opportunity' for access to the farm, Muriel's daughter, Diane McKay, has written to the Hertfordshire coroner asking for help.

She wrote: "We understand it is frustrating for the land owner to have my mother buried in his property, however he has been unwilling to cooperate and allow the family to search the correct location which Mr Hosein has led us to.

"As we understand, the family are entitled to my mother's remains and seek your help in accessing the property and exercising our common law rights to collect her and lay her to rest properly.

"This search has been ongoing for over two years and we are desperate to get closure."

Mr Marsh has been asked for a response.