They hope their meeting with Nizamodeen Hosein will help persuade the Home Office to let him return to Britain, so he can show them exactly where he buried Muriel McKay more than 50 years ago.

The move comes after police viewed Sky News footage of Hosein identifying the burial site to Mrs McKay's family on their recent trip to his native Trinidad.

Now detectives are preparing to make a similar visit to the freed killer, who says he wants to help recover Mrs McKay's remains and clear his own conscience before he dies.

Detective Superintendent Katherine Goodwin told the family: "I am hopeful that shortly after this visit we will be in a position to clearly seek an agreement with the landowners [if we have sufficient detail] or to apply to the Home Office.

"In the meantime, we are also making arrangements with our forensic and search teams so that they are prepared."

Hosein, now 76, and his older brother Arthur kidnapped Mrs McKay in 1969 and held her at their rundown Hertfordshire farm as they demanded a £1m ransom.

Hosein has claimed Mrs McKay, 55, collapsed and died while watching a TV news report on her kidnap, and he buried her on the farm.

Two weeks ago, in an emotional meeting with Mrs McKay's daughter Dianne McKay in a Trinidad hotel, he said he had panicked and carried the victim's body outside to the rear of a barn.

Stabbing his finger at a blown-up, black-and-white picture of the farm as it was then, he told Mrs McKay's daughter: "Two feet from the hedge, that's where the body is. If I go there I can find the exact spot."

Mrs McKay was the wife of newspaper executive Alick McKay, deputy to media mogul Rupert Murdoch. The bungling brothers mistook her for Murdoch's then wife Anna.

Read more:
Muriel McKay - the woman who vanished
McKay's family offer landowner £40,000 to search for body

The search for her remains was reignited three years ago when a documentary maker tracked down Hosein to his native Trinidad, where he was deported after serving 20 years for Mrs McKay's kidnap and murder.

It was one of the first murder convictions without evidence of a body.

After several phone calls with Hosein, Dianne McKay and her businessman son Mark Dyer decided they needed to meet him face-to-face to ask for his help, so flew 4,500 miles to the Trinidad capital Port of Spain.

Police believe Sky News footage of the meeting has given them a clearer idea of where they could start a new dig on the farm in the village of Stocking Pelham.

They excavated a large patch of land there two years ago and found nothing, but the family has said they searched in the wrong place.

The current farm owner banker Ian Marsh recently lifted his objection to a second search by saying that if detectives felt they had enough new evidence he would let them dig again without a search warrant.

Mr Dyer said: "I don't know that the police will get as much out of Nizam as we did, but it's a positive move that they want to go and see him.

"If it helps persuade the Home Office to let him come back to the farm, that would be great news.

"We need everyone there, especially the perpetrator, if we are to find my grandmother and bring her home."