A man who carried out “a ferocious and sustained knife attack on a defenceless old lady in her own home” has been jailed for life for murder.

The body of retired postmistress Una Crown, 86, was found at her bungalow in Wisbech, Cambridgeshire, on 13 January 2013. She had been stabbed four times and her throat had been cut.

Cambridgeshire Police did not treat her death as suspicious until two days later, but neighbour David Newton, 70, who was previously arrested but then released, was charged last April after a DNA breakthrough.

Judge Mr Justice Garnham told Newton that “Mrs Crown must have been utterly terrified when you came into her house”. He said he must serve at least 21 years in prison.

There was an intake of breath and whispers of “yes” from the public gallery at Cambridge Crown Court as the sentence was passed, and the defendant appeared to raise an eyebrow before being swiftly led to the cells.

Ahead of the court hearing, Mrs Crown’s niece Judy Payne told the BBC she “could finally smile again” following Newton’s conviction.

Following sentencing, in a statement read outside court by Det Sgt Dan Harper, the family of Mrs Crown said they could now “carry on with our lives knowing justice has been done”.

Mrs Crown’s body had been discovered in the hallway of her home by relatives who had arrived to take her to Sunday lunch.

However, police officers initially treated the death as “unexplained”. It meant police, firefighters, paramedics and undertakers walked around the house and some even touched Mrs Crown’s body to move things around.

It was not until two days later – after a post-mortem examination – that the death was finally deemed suspicious.

Police determined that Mrs Crown had died the night before her body was found.

Cambridgeshire Police admitted that “mistakes were made during the initial investigation in 2013” and it had apologised to Mrs Crown’s family.

However, DNA taken from Mrs Crown’s fingernails at the time of her death was retested by forensic scientists 10 years on, which led to Newton’s charge.

At the time he had been an odd-job man, living in Magazine Close a few streets away.

During his trial, the court heard that Newton had taken money from Mrs Crown’s handbag.

He had visited Wisbech Ex-Servicemen’s Club once on 12 January – when he played the fruit machine – and twice on 13 January, when he was “spending freely”, prosecutor John Price KC said.

Newton also paid £80 into an account on 14 January – the first cash paid in for 15 months, said the prosecutor.

Following his trial, Newton was found guilty of her murder by a majority of 10 to two jurors.

During the sentencing hearing, barrister Claire Matthews, from the prosecution team, said Mrs Crown had been stabbed with a “very long” knife – of at least 24cm (9.5in) in length. It was believed to be a kitchen knife.

Newton had attempted to destroy her body by setting fire to her – leaving her partially burned – and he had used the knife and Mrs Crown’s walking stick in the attack, she said.

There was no evidence of a sexual assault, she added.

Miss Matthews said the evidence suggested Newton was intoxicated and a “trespasser” in the house, she said.

Passing sentence, the judge said Mrs Crown would have “endured significant mental and physical suffering prior to her death”.

He also made reference to the police investigation of the case.

The judge said: “The first investigation was mishandled.

“Cambridgeshire Constabulary has, quite rightly, apologised to Mrs Crown’s family for that incompetence.”

However, Mr Justice Garnham continued: “After a disastrous start this was an impressive police and CPS [Crown Prosecution Service] investigation.”

He added: “I want to thank the family of Mrs Crown for the dignified manner in which they have conducted themselves, during what must have been an entirely harrowing experience.”

In mitigation, defence barrister Henry Grunwald said Newton was now “a very different man” to the man he had been in 2013.

“He is not a well man,” Mr Grunwald said.

Newton suffered from problems including depression, diabetes and rheumatoid arthritis, and Mr Grunwald added that his client could die in prison.