Now, in a deeply personal Panorama film, she uncovers the truth about assisted dying, meeting people with illnesses like hers who are desperate to choose the time and place of their death, and exploring whether British law should be changed to allow them to do just that.
Among those Margo meets is John Bowman, whose mother died with his father's assistance in 1999 after a 10-year battle with Parkinson's.
Until Mr Bowman's father had himself died, the family kept the truth of her death secret, aware of the potential punishment awaiting those who assist with suicide.
Mr Bowman recounts how his parents had sought "advice from like-minded people", then used drugs to suppress his mother's breathing and a special bag obtained from Canada to asphyxiate her.
These suicide bags, or exit hoods as they are sometimes known, are available online.
To investigate further Margo buys one herself, an experience which brings home the awful reality of the desperation some people face; people like Val Mackay, who is suffering from the most aggressive form of Multiple Sclerosis.
Dan James was one of many Britons who have travelled to Dignitas
"My son has had to help me to the loo, and that just shouldn't be," Mrs Mackay explains to Margo. "I mean, I have no privacy - none at all."
Mrs Mackay says that she has now reached a stage where she wants to end her life with medical help: "I had 40 odd years of a normal life. It's finished, it's over... and why prolong it in agony?"
Mrs Mackay's best option is the Dignitas Clinic in Switzerland, which will help foreign patients commit suicide. Nearly 100 Britons have already been to the clinic, and almost 700 have plans to do so in the future.
But for Mrs Mackay there is a problem, Dignitas requires the patient to be able to lift a cup containing a lethal dose and swallow it themselves - without assistance.
And though Mrs Mackay would be able to hold the cup now, that could quickly change, so she is faced with the prospect of curtailing what time she has left to ensure she arrives at the clinic able to perform this final task.
Mrs Mackay is in favour of the legalisation of euthanasia, where a doctor administers the patient with an injection. However, as Margo finds, among the key groups opposed to a change in the law are a large number of doctors.
Euthanasia has been legalised in the Netherlands
Margo talks to Sheila McGettrick, a hospice doctor who deals with people in the end stages of life, and is concerned that legalised assisted suicide or euthanasia could lead to vulnerable people being bullied into dying.
But in the Netherlands, where euthanasia and assisted suicide was legalised eight years ago, Margot is told that the notion of medics playing executioner is an incorrect idea.
Dr Rob Jonquiere, one of the architects of the Dutch euthanasia legislation, says that though the doctor is the one administering the injection, it is not they who makes the final decision:
"Just before he puts in the needle or he pushes the syringe, he will look his patient in the eyes and say, 'Well is this really what you want?' If the patient says I am hesitating he will immediately stop the whole process."
Back in the UK, Margo meets another doctor prepared to put a patient to death, but for whom the circumstances are very different.
Dr Michael Irwin was struck off after admitting being prepared to help a friend die by giving him a lethal overdose, even though his friend died of natural causes before the plan was put into practice.
Dr Irwin, who has taken several Britons to Dignitas and admits to helping patients to die here in the UK, argues that being prepared to help a patient die is an essential part of being a good doctor:
"Whenever I've helped people to die, I've been generous with diamorphine or have helped them to go to Zurich, I've felt that that was the compassionate act of a doctor - doing what a patient generally wants who is in desperate need of that final kind of help that only a doctor can give," he says.
Panorama: I'll die when I choose - Monday 8 December - 8.30pm on BBC One