Search YouTube for videos of Pete Doherty and you'll find a few with sizeable view counts. The Libertines' "Don't Look Back Into the Sun" – 12 million views. "Fuck Forever", Doherty's 2005 hit with his pin money side project Babyshambles – 4 million views. But there's another video that's had far fewer eyes on it: CCTV footage shot on Romford Street, Whitechapel, London, back in December of 2006. This one has been watched just over 34,400 times.

It's black-and-white and it's grainy, but in it you can see Pete Doherty and his 19-year-old then-girlfriend, Kate Russell-Pavier, running from the flat of Paul Nicholas Roundhill. That evening, Roundhill – often described as Doherty's "literary agent", but more commonly known for hawking the "blood art" Doherty has been known to produce with used syringes – had thrown a party. A small one. Just six people were present.

Roundhill's flat was known as a drug haunt. Visitors nicknamed it the "Hotel in the Sky", and Roundhill himself has admitted that people viewed it as a crack den.

In the video, as Doherty and Russell-Pavier leave the flat, they swerve a body lying prone in the gutter. Behind them lumbers Jonathan Jeannevol, AKA Johnny Headlock – in those days, Doherty's minder. The body is that of 30-year-old Mark Blanco.

Seconds earlier, the video shows the Cambridge-educated actor falling from the first floor flat's balcony to the street outside, a distance of 11 feet and seven inches. Unconscious and drenched in blood, it was only after another guest left the party that the Trinity College Philosophy graduate's body was discovered. Blanco died in the Royal London hospital that night of severe head injuries. Doherty, Russell-Pavier and Jeannevol have all denied any involvement in his death, and none have been charged with any offence in relation to it.

Sheila Blanco. Screenshot: BBC

"My son was exuberant," says Sheila Blanco today, when asked the question of who her son was. "He was full of life, a great wit. He read from a very early age, so was a precocious child. He loved the great literary tradition of the West and collected antiquarian books. He was a close-up card magician. He was a business consultant. He didn't suffer fools, but was not malicious. He was always generous and, at times, earned a lot of money. He'd worked for Goldman Sachs, but hated it."

Sheila – now in her mid-sixties, a piano teacher and English teacher to foreign students – threw out most of her son's clothes after he died. She knew that if she didn't do it there and then she wouldn't be able to do it at all. She kept his slippers, though, "to remind me of the way he walked".

She continues: "Mark lived in Paris for two years. He'd lived in Australia. He'd lived in Salamanca. He was a pacifist. He had friends from all walks… and he suffered from extreme vertigo. He would never have gone near the edge of those balconies."

Blanco had been due to appear in a performance of the play Accidental Death of an Anarchist, three days after his death. Written by Italian playwright and left-wing activist Dario Fo, it was to be staged at The George Tavern in Stepney, one of his favourite haunts. Blanco had been cast in the Anarchist role. Even today, a picture of him is pinned behind the bar.

Fo's farce is based on real-life events surrounding the death of Italian anarchist Giuseppe Pinelli, then-secretary of the Italian branch of the Anarchist Black Cross. Pinelli was accused of bombing the Piazza Fontana bank on the 12th of December, 1969. Three days later, Pinelli was seen to fall to his death from the fourth floor window of a Milan Police Station, where he had been taken for questioning. Posthumously, he was cleared.

Pete Doherty in October of 2006. Photo: Allstar Picture Library / Alamy Stock Photo

Blanco attended the party at Paul Roundhill's that night with a view to persuading Pete Doherty to attend the performance of his play. He'd heard Doherty was present from another drinker in The George Tavern. Reports say Blanco was "excited". That "he'd had a lot to drink". Roundhill would say that Blanco was "getting out of order". In time, Roundhill admitted to punching Blanco in the face three times, setting fire to his tweed cap and "dragging him out of the flat" because "he wouldn't leave and was being aggressive".

CCTV shows Blanco returning to the party a few minutes later. He was on the floor just 56 seconds after he'd re-entered the flat. Doherty, Russell-Pavier and Headlock left the flat 15 seconds later. They got into a taxi to the Malmaison hotel in Clerkenwell. Doherty trashed his room, then fled when the police were called at 2.25AM. Somewhere in this timeframe, Headlock has admitted to stabbing Babyshambles guitarist Mick Whitnall five times with a fork.

Between arriving at the party and lying dead in the hospital, Mark Blanco lost his glasses and his £800 TAG Heuer watch. "His glasses and his watch were missing when his possessions were returned to me," says Sheila. "A police officer had put them in his locker, gone on leave for 11 days and forgotten about them. This was one of the complaints that went before the Independent Police Complaints Commission."

Sheila Blanco visited the scene of her son's death the following day. She was horrified to learn that Limehouse police hadn't adequately cordoned off the crime scene. She found one of the lenses from her son's glasses in the gutter. In due course, Sheila would take many complaints to the IPCC.

Three weeks after his death, on Christmas morning, 2006, Johnny Headlock walked into Bethnal Green police station and confessed to the murder of Mark Blanco. Hours later, he retracted his statement. He cited stress. He told Newsnight in 2012, "The reason why I said to the police that I did it is because I'd go out and people would say, 'You murderer,' and the stress was just too much."

In 2014, in a hallway of Stratford magistrates court, Headlock twice told Sheila Blanco: "I didn't kill your son."

Now estranged from Doherty, Headlock reportedly had a stroke just months after his confession.

The coroner, Dr Andrew Reid, had long ruled out suicide. In 2012, after the Crown Prosecution Service had declared there was insufficient evidence to prosecute any individual, forensic scientists from the UK and the US, hired by Sheila Blanco, stated their belief that Mark Blanco hadn't slipped, hadn't fallen, but was most likely "dropped".

Of the CCTV footage, forensic expert John Kennedy told the BBC: "There's no defensive movement whatsoever. It's literally a drop from outside the rail." Grant Fredericks, a teacher at the FBI National Academy, agreed that the images of Mark Blanco falling are "very consistent with a person being carried and literally dropped over the edge of a balcony".

"We heard a massive struggle. Paul [Roundhill] was shouting, and there was lots of swearing," an unnamed neighbour told The Evening Standard a couple of days after Blanco's death. "There was a scream, and the guy landed on the floor. Two girls went down to the street to try to revive him. They called out to Paul to call an ambulance, Paul threw out some tissues and said, 'Sorry Mark, sorry Mark.'"

Three police investigations have failed to determine what happened to Sheila Blanco's son, though she is critical of each of them. "Corruption, ineptitude and gross inefficiency," she says in summary. "Officers at the crime scene were more interested in Doherty than conducting an actual investigation. There was an immediate assumption that Mark had committed suicide."

She continues: "Only three witnesses out of six who'd been present that night were called to the inquest from that flat. Headlock's confession was omitted from the police report to the coroner. Mark's clothes have never undergone forensic examination." She pauses. "We just expect police to be professional and do the job for which they are paid."

As well as Doherty, Headlock, Russell-Pavier and Roundhill, also present that night were Annabel Heald Smith, a friend of Roundhill's, and Naomi Stirk, who now uses her maiden name, Wang. Sheila has written to Kate Russell-Pavier. She received no response. She's spoken to Naomi Wang, who says she wants to "help get the truth". In February of 2007, Sheila turned up unannounced to Roundhill's flat and asked him what happened. "He told me he'd punched Mark in the face three times," she says. "He's a disgusting specimen."

Roundhill doesn't believe Mark Blanco's death was an accident, though his theory is best described as fringe. Roundhill believes the subject of the play that Mark Blanco came to his flat to promote is relevant in understanding what happened that night. He thinks Blanco was "making a creative statement in his mind, having been ejected, by jumping". (Incidentally, the musician Paul Cunniffe died in August of 2001 after a fall, also from Paul Roundhill's flat.) In a 2011 blog post, Doherty concurred, writing: "When he fell or jumped he was alone. That is what happened. He either fell or was making an extreme point about the nature of life imitating art."

Less than six months after his death, Doherty audaciously returned to where Mark Blanco died, to record a promotional video for the Babyshambles song "The Lost Art of Murder", a reference to the 1827 essay On Murder Considered as One of the Fine Arts, by Thomas De Quincey – one of Blanco's favourite writers.

Twelve years on, Sheila Blanco continues to fight for the truth about what happened to her son. As time has passed, more people have come to her with information, many with significant leads. But she needs more. She needs "just that little bit more".