Sheila Blanco: 'I've kept his slippers. They remind me of the way he walked'

'This is not about Pete Doherty, it's about getting justice for my son," says Sheila Blanco. Sheila is the mother of 30-year-old Mark Blanco, who was found dead having fallen nearly 12 feet off a balcony during a party in a flat in east London, four and a half years ago. She has had enough of being told she has it in for Doherty in particular, one of the people who was with her son that night. Last week, the Crown Prosecution Service ruled that there was insufficient evidence to charge anyone over his death.

This is a case that will not go away. Some will keep it going because Pete Doherty, the former Libertines frontman, was there that night, and was captured on camera stepping over Mark's unconscious body, before the ambulance arrived.

Others, like Sheila, a music and English teacher in her mid-60s, will keep it going, believing someone must be held to account, because that's what mothers do.

Nobody is entirely sure what happened in the seconds before Blanco hit the ground, but what is known leaves no shortage of loose ends.

Doherty's minder, Johnny "Headlock" Jeannevol, confessed to Mark's murder on Christmas morning 2006, but retracted his statement hours later. Party host, Paul Roundhill, sometimes described as Doherty's literary agent, admitted during cross-examination at the inquest in 2007 to striking Mark three times while trying to eject him from his flat.

I first spoke to Mrs Blanco and Mark's younger sister Emma, 29, for The Independent on Sunday a week after Mark died. Four and a half years later, now thinner and very tired, Mrs Blanco is more convinced than ever that her son, a Cambridge philosophy graduate, antiquarian book seller and actor, died after being deliberately pushed off the balcony and falling 11ft 7inches to the ground.

Not normally the emotional type, she is a practical woman on a mission for justice. Grief feels like a luxury she cannot afford.

"I can't really sit down and think about Mark very much as he was, because then that clouds what I'm doing. I have to concentrate and get the case on the road and get justice – that's all I want – and I have to be single-minded in that. After Christmas, I got very tearful and started to cry a lot about Mark, but I just felt terribly tired, I couldn't do anything... With this sort of thing, if you're too emotional, you can collapse and never recover."

Born in Lewes, Sussex, Mrs Blanco trained as a pianist at the Guildhall School of Music & Drama in London. It was in the capital that she met her first husband, Antonio Blanco Santiago, a Spanish accountant.

Mark was born in 1975, his sister, Emma, a professional violinist, five years later. She now teaches English to foreign students at Guildford College, and piano to students who come to her home.

The house where the family moved 25 years ago is packed full of Mark's beloved books, including the first copy of Little Dorrit by Charles Dickens, records, cassettes and, increasingly, files of documents Sheila has gathered as evidence. The morning sun fills the dining room, reflecting off the photographs of Mark – as a newborn, a floppy-haired toddler and as a young bohemian with thick-rimmed glasses – which are dotted around.

She got rid of most of his clothes soon after he died, somehow knowing that if she didn't do it then she probably never could. "I've kept a pair of his slippers. He had terribly smelly feet but that's not why," she laughs. "It's the shape of them. They're quite worn, so they remind me of the way he walked.

"And I've got his teddy bear, his Pooh, which he only stopped taking with him when he was 24."

Sleep has not come easily since being woken at 3am on 3 December 2006 with the shock of her life. For the next couple of years she awoke at the same time every day, as if expecting a phone call telling her it was all a mistake. Now, she wakes up every hour and a half, all night, every night. The determination and drive that keep her going in the day keep her awake at night.

Suppressing grief, or any strong emotion, has physical and psychological consequences: "I get flashbacks all the time; it's always in the back of my mind. There's kind of an aura, and sometimes I hear him speak, he had a very distinctive voice. My sleep is very disturbed. I have nightmares and strange images... About two or three weeks ago I heard him say 'Mummy, I know you're doing your best.' That affected me terribly all day."

And, with Doherty's history of drug use and consequent frequent appearances in the media, this means the Blanco family are regularly faced with one of the men they believe knows what happened to Mark.

But she is most angry at the police for failing, she believes, to properly gather the evidence or even examine the evidence she has tirelessly gathered by knocking on doors, hunting down witnesses, and hiring experts. "I am not asking for the impossible, I am just asking for justice... Somehow or other, the truth will come out."

The next steps will inevitably include a complaint to the Independent Police Complaints Commission, to try and ascertain why, for example, the confession by Headlock was never submitted to the coroner's court; why Doherty was not interviewed by police until three months ago; and why Mark's clothes were never sent for forensic examination.

After that it could come to a civil action. She is weighing up her options with her barrister, Michael Wolkind QC, who continues to represent her, free, equally convinced that justice has been so far denied. There is no question, not even the slightest flicker of doubt, of giving up the fight. She tries not to burden Emma with her mission for justice, because she doesn't want it to take over her life, and their relationship, as well.

Sheila Blanco knows that at some point she must allow herself to stop, collapse in a heap and weep for as long as she needs to. But for now, sadness is not something she is willing to give in to.

"I've grown up [since he died] and realised that the people we believe in, well perhaps we shouldn't. Mark was so right about the establishment; he's been vindicated in death... I can't stop because this is a miscarriage of justice. No more, no less. It's a basic human right to have a thorough investigation, Mark didn't have that and I want to know why."