As the singer makes another comeback with the Libertines, BBC reporter Peter Marshall investigates the case of Mark Blanco and the new digital evidence that could finally resolve an eight-year mystery

Johnny Headlock is suddenly there, right in her face, determined to say his piece. "I didn't kill your son." He waits a few seconds and, getting no response to the dramatic proclamation of innocence he made in the hallway of Stratford magistrates court, he repeats: "I didn't kill your son."

Sheila Blanco appraises the man who was one of the last to see her son Mark alive and turns away. She can barely count the number of times, in the last eight years, that she has seen the CCTV footage of Headlock leaving the scene with the rock star Pete Doherty, his employer, while Mark Blanco lay dying. He had fallen from a first-floor balcony in a block of flats in London's East End after attending a party with Doherty. The cause of his death, in suspicious circumstances in 2006, has never been resolved.

Headlock's extraordinary confrontation with Sheila Blanco, unreported until now, happened when he appeared in court on an unrelated matter earlier this year. These days Headlock is in poor health. He's said to have suffered a stroke and is no longer the physically intimidating figure who was Doherty's minder around the boho rock scene of nearly a decade ago.

Meanwhile, Doherty – preparing for the relaunch of his band, the Libertines, at a concert in Hyde Park next weekend and a European tour from which he is expected to make millions – is now refusing to discuss the case or why he ran away.

Headlock is no longer a member of Doherty's inner circle, but other old pals are gathering. Among the 65,000 crowd, with a VIP pass, will be Paul Roundhill. "Yes I'll be there. I've got tickets for the Summer Garden," he says proudly, adding he's still close to Doherty. "I was over in Hamburg a couple of weeks ago, for his video."

It was in Roundhill's Whitechapel flat that Mark Blanco met Doherty in December 2006. Roundhill describes the flat as an intellectual salon but concedes others saw it as a crack den. Blanco wanted to persuade Doherty to see his forthcoming appearance in a play, but he was deemed such a nuisance that the pop star asked Headlock to "have a word".

Roundhill admits forcing Blanco out of the flat and punching him in the face three times – he returned to the building shortly afterwards, though people in the flat say they were unaware he was back. But Roundhill, Headlock and Doherty all deny knowing how Blanco came to lie dying on the pavement after going over the stairwell balcony.

The initial police investigation was slipshod: they did not seal off the scene for forensics and failed to tell the coroner of Roundhill's punches or of how Headlock had walked into Bethnal Green police station and confessed to killing Blanco – a confession he had also made to others and which he subsequently retracted, saying he was under stress.

Detective Inspector Mark Dunne told Mrs Blanco he was "98% certain" the case was a suicide – his brother had died in the same way. An internal police investigation later concluded Dunne was trying to empathise with the family and this had not clouded his judgment.

The Independent Police Complaints Commission subsequently took a different view, upholding Sheila Blanco's complaint that "DI Dunne made unqualified presumptions into the cause of Mark Blanco's death thereby failing in his duty to pursue all reasonable lines of inquiry".

Summing up at the inquest, the coroner, Dr Andrew Reid, said: "I will no longer go on any further to consider the possibility that Mr Blanco committed suicide, I can exclude it unreservedly." He recorded an open verdict and asked the police to reinvestigate.

They eventually reported that, given the poor quality of the CCTV footage, the death would probably remain a mystery. But expert analysis I commissioned for the BBC's Newsnight programme in 2012 suggested vital clues had been missed on the CCTV. John Kennedy, a pioneer of video forensics in Europe, said these indicated Blanco may have been unconscious before he fell. And Grant Fredericks, who lectures in video science to the FBI, said the pictures strongly indicate at least one other person appeared to be with him on the balcony.

Since then Sheila Blanco has been increasingly frustrated by the snail's-paced progress of the police investigation. "It's almost as if they don't want to find the truth. We've had to spoon-feed them with evidence from the start and still they don't act," she says.

It took another year for the police to submit the video discs for re-examination. Now, after a further seven months' delay, they've still not organised the reconstruction, recommended by both Fredericks and Kennedy, to finally establish the truth.

Fredericks, in the UK last week to lecture to police and scientists at a seminar in Coventry, said he was "puzzled" by the Metropolitan police's position. "The Met haven't been charged with anything so don't need to be defensive yet they seem to have taken an adversarial position. Instead of being open to discuss the science, they've circled the wagons."

After visiting the scene of Blanco's death, Fredericks said he was more convinced than ever that a process known as 3D reverse projection would establish the truth. It involves recreating the exact vantage point of the CCTV camera and measuring various movements against the original video material. Fredericks and Kennedy used it most dramatically to establish the innocence of a man wrongly imprisoned for murder in Bedford in 2010.

"You can analyse 250 million measurement points," said Fredericks, adding that to apply it to the Blanco CCTV "would take less than two hours, maximum". Sheila Blanco's solicitor, James Saunders, said: "We've done the difficult work and found world experts with unimpeachable credentials. What are the Met waiting for?"

It was Saunders who tipped off the Met that Johnny Headlock, under his real name Jonathan Jeannevol, was to appear before magistrates in February on an unrelated stalking charge. His victim, Naomi Stirk – with whom he had once been intimate – said she was terrified after he had turned up outside her home and sent her more than 100 unwanted texts, some of them threatening.

Stirk is also a witness in the Blanco investigation. She says she saw him being bullied and removed from the Whitechapel flat before he died in 2006.

Watched by Sheila Blanco in the public gallery, Headlock was issued with a restraining order by Stratford magistrates forbidding him from making contact with Stirk. They ruled he should not face a trial for stalking after they received his psychiatric reports.

Headlock had appeared distracted in earlier hearings, shouting at one point, "they think I killed Mark Blanco".

Within six months of Blanco's death, Doherty returned to the Whitechapel flat to make a video promoting his song, The Lost Art of Murder. He later told a music journalist that he believed Blanco must have jumped from the balcony as "an artistic statement".

Blanco had been due to appear in The Accidental Death of an Anarchist, a play by Dario Fo where the protagonist falls or is thrown to his death from the window of a police station.

In an interview with the NME in 2012, Doherty claimed the police understood he was innocent. "The murder squad down at Limehouse, yeah, they opened and closed that case three or four times. The pressure they put on me was completely minimal. And they kind of said off-tape 'Pete, we're sorry about this, right, but the family are convinced that their son was murdered'. I can understand it and it does look dodgy."

Sheila Blanco describes Doherty's attitude as shocking. "He's never apologised to me, not a word. He was there. Mark upset him. Doherty actually stepped over my son's body! Yet for him life goes on as though nothing happened."

Sheila Blanco does not know exactly who was responsible for her son's death. She has her theories but no absolute certainties. But she is clear about those she feels could have done more to help her get to the truth. It's a list that begins with Doherty and ends with the Metropolitan police. Strange bedfellows in a protracted and disturbing tragedy.