Nizamodeen Hosein had kept the secret of Muriel McKay's fate for more than 50 years, long after his release from prison, until her family made him an offer he couldn't refuse.

But after signing an agreement that would have rescued him from a life of poverty, he turned down the money (the equivalent of £40,000) and told the family how and where Muriel died.

He has now offered to return to the UK, from where he was deported at the end of his life sentence in 1990, to show Mrs McKay's daughter Dianne and her grandson Mark Dyer the site of Muriel's remains.

Hosein claims she died from a heart attack at a Hertfordshire farm owned by his brother Arthur a few days after they kidnapped and held her for a £1m ransom at Christmas in 1969.

At his squalid, remote home in Trinidad he explained to the family's lawyer: "I don't want the money. Money wasn't my objective, it was peace of mind.

"Talking about it now breaks my heart. I was young, 22, I didn't feel the pain like I'm feeling now."

Mr Dyer, a businessman who drew up the contract with lawyers, said: "It may seem odd to many people that we should pay Nizam Hosein for the information, but our offer unlocked everything after many years of his silence and our sadness and frustration.

"It seemed our last chance of ever finding out what happened to my grandmother.

"Nizam could certainly have made good use of the cash because he is living in a hut with rotting floorboards, no proper sanitation and poisonous snails climbing the walls.

"He seems to have rejected the money because he wants closure. He's getting old and he's frail and it was perhaps his chance to atone for what he did.

"Our lawyer gave him the first $500 and he just pushed it away. For me, that gave him legitimacy."

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Muriel was aged 55 and the wife of newspaper executive Alick McKay, deputy to press baron Rupert Murdoch - who had just bought the Sun and News of the World.

The bungling brothers mistook Muriel for Murdoch's first wife Anna after following the wrong car, Murdoch's Rolls Royce, to the McKay home in Wimbledon, South London.

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After days of playing cat and mouse with the police - and a bodged attempt to pick up a suitcase of ransom money - the kidnappers were caught and arrested at the farm.

However, there was no sign of Muriel and they refused to say what had happened to her.

They were jailed for life after one of the first murder convictions without the discovery of the victim's body. Arthur Hosein died in prison in 2009.

Last year, Scotland Yard searched part of the farmland near the village of Stocking Pelham after talking to Hosein on a video link, but he later insisted they had dug in the wrong place.

Some of the farm buildings, fencing and gates have been changed in the intervening years since the murder, which gripped the public and made worldwide headlines at the time.

Detectives have sent Hosein a list of more than 80 questions in a bid to check his story and narrow down the true burial site.

They are in contact with the McKays and considering applying for a warrant for a new search at the farm.

The family has launched a petition calling on the Home Office to lift Hosein's deportation order temporarily so he can revisit the farm.

He said: "If I go back to the farm, I will remember where I put the body. I am sure I can go to the spot directly."