Three years ago, Sheila Blanco was not a vengeful woman. She had a job she loved, teaching students music and English at Guildford College, and was looking forward to spending time in northern Spain where her son Mark, an actor and writer, was planning to move.

But on 3 December 2006 that all changed. Mark, a 30-year-old Cambridge graduate, died after falling from a balcony of a flat in Whitechapel, East London, during a party attended by, among others, the singer Pete Doherty – who Mark was trying to convince to go to a play in which he was starring. An argument ensued and Mrs Blanco, who believes her son did not fall but was pushed, began a fight for justice. And she is still fighting.

Well-spoken and impeccably dressed, she appears stern, reserved and detached – even a little cold. "I hardly knew who Pete Doherty was," she tells The Independent with a calm assurance. "I was vaguely aware that he was a musician, but this man came into my life, destroyed it, then skipped off into the sunset. He stuck two fingers up at everything I held dear.

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"Whether it is the master, Doherty; the dog, Johnny 'Headlock' Jeannevol; or the weasel in the middle, Paul Roundhill, they are all culpable in some way for my son's death," she says of the three men known to have clashed with Mark the night he died. Her determination to prove her son was murdered makes her a formidable foe.

In the early hours of that fateful morning in 2006, Doherty's minder, Jeannevol, had grappled with Mr Blanco and the singer's friend, Roundhill, had punched him in the face. Partygoers later said Mr Blanco was thrown out because his conduct was "socially unacceptable".

Less than a minute after returning to the flat where the party was held, apparently to collect some belongings lost in the struggle, Mr Blanco was lying on the pavement with fatal head injuries, having fallen from the balcony. A coroner recorded an open verdict and ordered a second, independent, investigation.

Mrs Blanco says her faith in humanity has been shaken by the "callous actions" of the men she holds responsible for her son's death that night. "I am disgusted by human nature. I came from a difficult background, which I do not talk about, and I think there are evil people around."

She says she hates to hear parents whose children have died talking about how wonderful they were and says Mark was not special, even admitting that he was, at times, naïve.

"Mark was always going on to the next thing in his life and he trusted people to help him. He would never have believed they would turn on him like they did. He paid the price for his trust. But I think he deserves justice in death, as anybody does.

"Mine is a bigger fight than just for one person. I have beliefs and this is a democratic country; if we want changes to be made, we need to make them ourselves. The people of this country are owed a proper investigation and I consider myself lucky to be able to speak out," she says. Mrs Blanco balks at the idea of easing up in her fight. "It is incredibly tiring but I can honestly say I have never woken up in the morning and wanted to take a day off," she adds.

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"I have not sat down and cried for a long time because if the adrenaline did not keep me going I would not be able to cope. There is only me to deal with this. I cannot burden my daughter, I have to be the one to push this fight forward. I simply fail to see why people should get away with murder."

In her three-year campaign to prove her son was murdered, Mrs Blanco has interviewed people who attended the party, as well as neighbours. Johnny Jeannevol initially owned up to killing Mr Blanco but later retracted his confession. Doherty and his cronies may not yet have faced Mrs Blanco but she believes their day of reckoning may be near. Recently, she alleged that expert evidence supported her claim that her son was pushed.

Professor Richard Wassersug, an expert in anatomy and neurobiology, and Corrina Cory, a security expert with the engineering company Arup, found the actor's injuries (almost exclusively to the head) were not those of someone who intended to fall or jump. The possibility of suicide was officially ruled out by the second investigation, leaving only two possible explanations: Mr Blanco jumped, not intending to hurt himself, or he did not mean to fall at all. In his report, Professor Wassersug wrote: "To sustain lethal injuries almost exclusively to the head in a fall from such a low height [11ft 7in] would require more than just drug-induced poor co-ordination and slow reflexes.

"Based on the research, I stand by my opinion that it is extremely unlikely that Mark jumped deliberately, but if he did he did not intend to harm himself. Given the nature of his injuries, the two most likely explanations are that he was backed into railings [in front of the balcony] and pushed over, or he was not conscious and was dropped over the railings."

A character reference from a friend of Mark's, Dr Anthe George, suggests that he would not even have been able to stand on the balcony of his own accord because of his acute acrophobia. "Mark was truly afraid of heights. I do not mean he was afraid of standing on the edge of a cliff – he was afraid of any height."

Mrs Blanco, who runs the website justiceformark.com, says she will not rest until it is proved that her son was murdered and his killer is brought to justice. And her steely glare leaves you in no doubt that she means it.