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And in his newly-published memoirs, A Difference of Opinion, Mr Sillars pays fulsome tribute to his wife and writes of the SNP leadership’s efforts to stymie her Scottish Parliament career.
When Mr Sillars was elected as a Labour MP in 1970 he was known as the “Hammer of the Nats” for his forceful attacks on the SNP but he describes how, through a process of “self-challenging” he gradually changed his views, founding the rebel Scottish Labour Party out of frustration at the weakness of Labour’s original devolution proposals, and eventually joining the SNP.
Margo burst on the scene as the "blonde bombshell" who won the 1973 Govan by-election for the SNP and quickly became one of the party's best-known figures. Mr Sillars says he was pleased when she lost the seat back to Labour at the general election a few months later, thinking that she would disappear. “I didn’t know her then. What I didn’t realise was that her best platform was not the Chamber of the House of Commons but the public meeting where she could develop her ideas and engage in discussion with people. Margo was very much a people person.”
He joined the SNP in 1980 and the following year he and Margo married and set up home at Balcarres Court in Morningside.
When the Scottish Parliament was being set up in 1999 Margo was keen to be part of it.
The first stage for SNP hopefuls was to get the endorsement of a vetting committee to get on a list of approved candidates.
Mr Sillars claims Margo would have been rejected at this stage if it had not been that Labour had just provoked a furore by vetoing the respected left-wing MP Dennis Canavan as a Labour candidate for the parliament. He says one committee member predicted an even worse furore if one of the country’s best-known politicians was blocked from standing, so she got through.
The next hurdle was being selected for a constituency and Margo was invited to go for Edinburgh South.
"Alex Salmond went all out to stop her, supporting a former senior civil servant he thought would beat her. By support I mean that he phoned round delegates to urge them not to vote for Margo. She won the selectionbote 25-0 and then it was 'Stop Margo' from being number one or two on the Lothian list. It failed."
Although she did not win Edinburgh South, as number one on the Lothian list Margo was elected as one of the original MSPs.
"Thereafter every effort was made by the party leadership to damn Margo and get her out of the parliament the next time. Alex had decided to go back to Westminster and John Swinney became leader but nothing had changed in respect of Margo."
He tells how Mr Swinney criticised Margo over comments in her Evening News column in 2002 about French National Front leader Jean-Marie Le Pen.
"There was an intention of forcing Margo out of the parliament because she was not toeing the party line and demonstrating too much independence. It was not well hidden and John Swinney's confected outrage was one sign of it."
When it came to ranking candidates for the 2003 Scottish Parliament elections, Margo was pushed from top slot to fifth on the SNP list, meaning she had no chance of being re-elected, so she decided to stand as an independent.
Mr Sillars claims the leadership tried to "sink her candidacy" by leaking the fact she had Parkinson's disease, which meant her eldest grandson learned of his granny's condition from a friend at primary school whose parents had read it in the papers. The move backfired and Margo was elected as an independent MSP. Mr Sillars writes: "In later years she was very forgiving of those who had leaked the story."
Mr Sillars tells how before the 2014 independence referendum it was clear Margo's Parkinson's had reached a stage where she could not campaign in person and she insisted he come out of retirement to campaign in her place. "I could not refuse that order."
But he goes on: "The cause for Margo was all. Her willpower was something extraordinary to witness. Ill as she was and dying, she got herself dressed and made up and did a long interview for the BBC in our home, with Jackie Bird, about her views on independence, including on Alex Salmond. It was a monumental physical and mental effort."
They had been told Margo had about three months to live, but it turned out to be just three weeks and she died on April 4, 2014, aged 70.
Mr Sillars was devastated. "We were two halves of one whole. We did not always agree and would often argue fiercely, but we sparked off each other and needed each other to function. I have never been the same since."
Despite her clashes with the leadership, Margo and Mr Salmond had once been allies in the left-wing 79 Group.
"In her last days, the one thing that pleased her most was a private visit from Alex Salmond. Her relationship with him, like mine, had deteriorated but her getting back onto the old footing one last time meant a very great deal to both of us."
And he recalls his tribute to Margo at the memorial for her in the Assembly Hall on The Mound, in which he appealed to people to adopt "the Margo MacDonald way" – “to realise that your are dealing with opponents, not enemies; not with ogres, but with fellow human beings, with whom you can disagree but do so without malice”.