MARGO MacDonald phones to say I should come to lunch at Holyrood. “Ask for me at the front desk, which cost £88,000. You’ll have seen similar out at Ikea.”

This is a typical Margo bon mot – a needling criticism of the expense of the Parliament building couched as a gentle witticism with a sure populist touch. She is simultaneously waspish and wee-wumminish, Dorothy Parker meets Dorothy Paul. Lunch, anyway, is a grand idea. I’m keen to learn more about our only independent MSP as she approaches both her 70th birthday and the 40th anniversary of her first election victory.

The official reason for meeting, though, is the Assisted Suicide (Scotland) Bill which she plans to introduce, perhaps as soon as next month. Two years ago, her bill which sought to give terminally ill people the right to choose when to die was defeated, but now she plans to try again with a revised proposal. “I couldn’t let it go,” she says. Too many people have written with dreadful stories of their own experiences of living with intolerable illnesses for her to give up now.

She has, besides, a personal reason for wanting this legislation passed. In 1996, she was diagnosed with Parkinson’s and wants the right to an assisted death at the time of her choice. She would rather not make too much of her own illness, because – she says – she doesn’t want to be held up as any kind of example, but also – it seems to me – because she has an aversion to being considered weak. She has a large bruise on her right forearm, the result of a recent fall, and says that, had she noticed, she should have worn a different top which would have hidden it. She worries, too, that people will see her trembling hands and think that her mind must be similarly shaky. “It gives the impression that I’m a wreck,” she says. “I’m not a wreck. Well, I am. But I’m not a wreck from here up.” She points from her chin to the top of her head. “Some people make that ­mistake.”

Life in politics, her husband Jim Sillars tells me later, is sustaining for MacDonald. He would never have dreamed of telling her to jack it in. He believes that the Parkinson’s would have progressed faster had she not been so busy. “This is who I am,” is how she puts it. She came to Holyrood in 1999 as an SNP member, but decided to stand as an independent in 2003 after being placed fifth on the party list for Lothians, an effective deselection. She was expelled from the SNP and has continued to be elected as an independent in 2007 and then again last year, a testimony to her popularity with the public.

We sit down in the members’ restaurant – “The chef who makes the sweets is a friend of mine. He’s a Jambo, but still a friend.” Here, she holds court, regally gallus in a black and pink kimono jacket and big gold jewellery, being ­visited at the table by politicians both Nationalist and Labour, as well as by a visiting member of the public who stops by to congratulate her on her contribution to First Minister’s Questions. My laying of two tiny voice recorders on the white tablecloth brings to her mind the period after she won the 1973 Govan by-election when, in short order, she was taken out to lunch by first the KGB and then the CIA, agents of whom were posing as journalists. “But,” she insists, “I never let them take me anywhere flash.”

Govan was extraordinary; a victory so unexpected that even the candidate herself – running a campaign out of a condemned building – didn’t see it coming. It established MacDonald, aged 29, as, in her own phrase, “the hottest ticket in the political firmament”. And it was a landmark win for the SNP, which at that time had only one other Westminster MP, a little glimpse of what might be possible: winning in a Labour heartland. “The nerve of what we did,” she says, shaking her head at the memory. She lost the seat in 1974, which she found devastating and took as a personal rejection, but by that point her name was made and the SNP, of which she became deputy leader, was enjoying a surge. Grandma Moses she calls herself jokingly, and ­certainly there is something in the idea that she moved her party a good bit closer to the electoral promised land.

In 2010, her End Of Life Assistance Bill was defeated by 85 votes to 16. She is more confident this time around, believing that the public and political mood has shifted significantly towards her point of view. If so, then the main reason, surely, is Tony Nicklinson, the sufferer of “locked-in syndrome” whose legal quest to secure an assisted suicide was denied at the High Court last month; a few days later, heartbroken and refusing antibiotics to treat pneumonia, he died. At the mention of Nicklinson, MacDonald begins to cry. “He was there on the television – no dignity, no privacy, proving that not everybody can be palliated. And that’s what the Bill’s for.”

Knowing that legal ­assisted suicide is an option, the argument goes, would be a comfort for people with terminal or intolerable illnesses. As Terry Pratchett has put it: “If I knew that I could die at any time I wanted, then suddenly every day would be as precious as a million pounds. If I knew that I could die, I would live.” MacDonald nods enthusiastically at this quote. “Yes,” she says. “I would feel much better about it. I would feel much better for my family, knowing that they didn’t have to go through a long, exhausting, very, very sad end to my life.”

Jim Sillars is completely supportive of his wife’s desire to end her life at the time of her choice. “I’ve got no problem with that because I’ve got the same desire. If something really serious ­happened to me, I would be away in two seconds flat.” He insists, though, that he would be unwilling to help her do so. “I’ve told her I’m not going to Saughton. I’m a bit old for that, unless they’ve got a very good library. I’m not prepared to deliver the pill, as it were.”

Theirs is a love story. Love and politics. On the day we meet they are preparing for a weekend at their holiday home in Portugal, where she will swim and he will sit by the pool reading the archives of the Soviet Communist Party. For Sillars, who was himself elected as a Labour MP in 1970 and then in Govan for the SNP in 1988, his wife is a figure to be admired as much as adored. “I have met three outstanding minds in my political career. One was Enoch Powell, who had a Rolls-Royce mind, and the other two were Jimmy Reid and Margo.”

MacDonald was born in 1943 and grew up in and around East Kilbride, one of three children. Her mother, Jean, was a nurse. And her late father, Robert? “He was inadequate. He was very cruel and my mum took us away for our own good.” More than this, she is not willing to say on the record. She was about 12 years old when her parents separated, so beginning a “desperately poor” and somewhat peripatetic period during which she lived, for a time, in a caravan and then with the family of a farmer who took them in. Stability came in her late teens with the provision of a corporation house, an end terrace in East Kilbride. It is possible to see in the struggles of her childhood the roots of MacDonald’s independence and self-sufficiency, personality traits which would later chime with her political philosophy. What she wanted for herself, she wanted for her nation. “If you have to rely on yourself, you try harder,” she says, “and when you try harder, you feel bigger.”

In 1965, she married her first husband, Peter MacDonald, and the two of them took over the running of the Blantyre pub he had inherited from his father. This was the Barnhill Tavern, known as The Hoolet’s Nest. “If you ask what shaped me, it was that as much as anything.” A pub landlady at just 21, MacDonald learned responsibility and how to talk to the men, many of them drouthy miners and steelworkers, who crowded the bar. Politics was much discussed. It was a heady time, internationally: the US civil rights movement; equal pay for women; in or out of Europe; the Prague spring; the Paris riots; Scottish independence – all of these and more were refracted through a pint glass.

Margo had two daughters, Petra and Zoe, but the marriage did not last. “I loved Peter with all my heart, but the pair of us fought like cat and dog ­because we were too young and had too much responsibility,” she says, in tears again.

Lunch over, we move downstairs, heading for a bar just off the garden lobby. MacDonald walks with sticks, which she hates, and uses a mobility scooter. She glides magisterially along the corridors of power, singing a snatch of Big Yellow Taxi, treating Holyrood as a catwalk show (“There’s a deputy presiding officer. I like her shoes”) and ­delighting in disrupting filmed interviews for the television news with the beep-beep of her reversal. She wants Willie Nelson’s Always On My Mind played at her funeral, she says in passing, having thought better of Bat Out Of Hell.

In the bar, over coffee, sitting in an area she calls Margo’s Snug, she seems a little tired. But she won’t stop talking. Not stopping is her modus operandi. Does she ever consider retirement? Will this be her last term of office?

“I can’t tell you that,” she says with a smile, rings and eyes flashing. “If you admit it, you’re a dead duck. Anyway, I’ve got things I want to do. That’s why I’m still here.”