A mother who waited 31 years to see her seven-year-old daughter’s killer brought to justice said she will take legal action against the police force which botched the initial murder inquiry.

In October 1992, schoolgirl Nikki Allan was lured to a derelict building in Sunderland and was stabbed and hit with bricks by David Boyd, now aged 55, and left dead or dying in the basement.

Northumbria Police charged innocent George Heron with the crime only for the trial to collapse when the confession they gained after three days of oppressive questioning was deemed inadmissible.

Nikki’s mother, Sharon Henderson, never gave up her fight for justice, made her own investigations and repeatedly called on the force to re-open the inquiry.

The battle came at a great cost to herself and her family, as she struggled with her mental health and drinking, and got into trouble with the law.

Following a DNA breakthrough and painstaking police work, Boyd was arrested and was convicted of Nikki’s murder at Newcastle Crown Court in May, and was sentenced to life in prison with a minimum term of 29 years.

Ms Henderson told the BBC’s Newsnight programme she will take legal action against the force, saying: “I was treated really badly by the police.

“Because I was the one parent, I didn’t have any support and I was drinking heavily.

“I was living in a council flat and I didn’t have any money.”

Boyd was the boyfriend of Nikki’s babysitter, lived in the same block of flats as her and told officers he had seen her on the night she disappeared.

He was never treated as a suspect, even after he was convicted of a child sex offence in the late 1990s, and remained at large until the DNA breakthrough in 2017.

After Boyd was sentenced, Harriet Wistrich, director at the Centre for Women’s Justice, said: “Now, Sharon demands that Northumbria police are held accountable for their historic failures that allowed a 31-year wait until the right man was convicted of Nikki’s murder.

“The full extent of the failures remain unknown and therefore we are demanding a public inquiry to examine the historic investigation which led to the wrong man being prosecuted and the police failing to consider alternative suspects until over 20 years later.”

After the jury convicted Boyd, Northumbria Police apologised to Ms Henderson, as well as to Mr Heron who had to leave Sunderland despite being cleared in 1993.

At the time, Detective Chief Superintendent Lisa Theaker, who led the successful inquiry, praised Nikki’s family, saying: “I would like to thank them for their patience and strength shown during their relentless pursuit of justice.”

Northumbria Police declined to comment about any potential legal action.

The force has offered Ms Henderson a meeting with Assistant Chief Constable Alastair Simpson, who issued the previous apology to the family and to Mr Heron.

Ms Wistrich, who is also Ms Henderson’s solicitor, has written to the chief constable, Vanessa Jardine, to inform her of the legal action; and to the Police and Crime Commissioner, Kim McGuinness, asking for a formal inquiry into the case.

Ms Wistrich said: “Sharon and her daughters have suffered immense pain and damage as a consequence of historic police failures.

“She never gave up on her attempts to secure justice for Nikki.

“Now the murderer has been convicted, she wants answers and a full inquiry into the historic failures by Northumbria Police.”