After a renewed campaign to find her body, her relatives now fear police will abandon plans to dig at the Hertfordshire farm where Mrs McKay was held ransom by her kidnappers 55 years ago.

Her family says the killer has already pinpointed the burial site to them.

The British officers collected Nizam Hosein, 76, from his ramshackle home in Trinidad last week and spent three days in a local police station asking him to identify the exact spot where he buried Mrs McKay.

Hosein was deported to the island after serving 20 years for Mrs McKay's kidnap and murder. It was one of the first murder trials without a body. Until recently he had refused to say what happened to his victim.

After initially telling the family they were making progress in their interviews with Hosein, Detective Superintendent Katherine Goodwin then sent them a message: "He was unable to provide a location with any consistency, which is not what you or we wanted to find."

Mark Dyer, Mrs McKay's grandson, confronted the officers on their return to Gatwick Airport early on Saturday.

He told Sky News: "This is most upsetting to us personally, having done so much for this search to find my grandmother who has now been twice failed by the Metropolitan Police.

"We warned the police that going mob-handed and putting him in a police station would spook him and they would never get much out of him. He is terrified of police officers and needs to be carefully handled and encouraged to speak about those days."

Dianne McKay, Muriel's daughter, said: "It's taken us nearly three years to get this guy to really be open and friendly with us, and that's not what we ever set out to achieve.

"We only wanted information, but we've had to work very hard psychologically on his character to gain his confidence, and they walked in and snuffed it."

Mr Dyer said: "Many times Nizam has told the family the precise burial spot. He hasn't wavered. He pointed it out on old photographs of the farm we showed him and has offered to return to the UK to show us exactly where we will find my grandmother."

He also told Sky News that he feels he and his mother "are being played with".

"It's not a game," he added. "My mother's emotions and health are being played with, this has got to stop.

"Either my grandmother is where Nizam says she is, or she's not, it's simple. This is not rocket science."

Businessman Mr Dyer and his mother Dianne met Hosein in January after flying 4,500 miles to Port of Spain, the capital of Trinidad and Tobago.

Sky News filmed a series of meetings, in which Hosein was shown old and new photographs of the farm and studied computer-generated images to identify the burial site.

He said at the time: "Go through the kitchen door, come through the open land, turn left and it's two feet from the hedge, that's where the body is."

A week later, after studying the Sky News footage, Det Supt Goodwin said she found Hosein's evidence "compelling", but wanted to meet him face to face to test his credibility and memory.

She and two colleagues landed on the island on Monday and began interviewing Hosein the next day. They had urged the family not to be there and to let them speak to him alone.

She hoped to gather enough evidence to justify a new search at the farm near the village of Stocking Pelham, or urge the Home Office to lift Hosein's deportation order and let him return briefly to the farm to show police exactly where to dig.

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Her colleagues searched a patch of the farmland two years ago, but found nothing during a five-day excavation. The family said they had dug in the wrong place.

Fifteen months ago, Dianne McKay, 84, made an official complaint about the attitude towards her by one of the officers involved in the first search.

She accused him of "completely and wholly unacceptable behaviour" by confronting and shouting at her and accusing her of breaching an agreement with the landowner who had allowed the first police search.

She wrote to Commissioner Sir Mark Rowley: "I was gravely surprised and still feel deeply traumatised by his behaviour."

Scotland Yard spokesperson said: "We can confirm a public complaint has been received and is now being assessed. We will remain in contact with the complainant during this process."