Then, last summer, the deaths were back in the news. Now a Sunday newspaper claimed Horse – artist, musician and award-winning children's writer – had "killed his terminally ill wife in a frenzied knife attack that left her with more than 30 wounds", after taking "a cocktail of drugs"; he then killed their pets before stabbing himself and bleeding to death. Shortly afterwards Mandy's father, George Williamson, was quoted as saying: "It was murder. She wasn't planning on suicide."
Among those most shocked by the story was actor Tam Dean Burn, who had recorded a radio tribute to Horse (whose real name was Richard Horne) in the week of his death, and was developing a one-man show based on his cartoons. "I thought, his reputation has been so destroyed there's no chance of doing a show," he recalls. So he decided to abandon the project.
And then, after talking to Horse's mother and sisters, he changed his mind. Less than a year on, he is about to stage the show – a tribute to the artist's work called Year of the Horse – bolstered by the belief that, while "we'll never know for certain" exactly what happened (the procurator fiscal ruled against holding a Fatal Accident Inquiry) that the description of Horse as a killer "running around in some mad frenzy" was a cruel distortion.
"Harry and Mandy had attempted an overdose and it hadn't worked, so they were taking it on to the next stage," he maintains. "This was a couple who were absolutely besotted with each other, and they had decided that they were going to go together. She was not long for this world, her MS was utterly crippling, and he'd decided – they'd decided – that he was going to go with her. It was an increasingly difficult situation. But they knew what they were doing, and I think they should be respected for that."
How will Mandy's family feel about statements like this, though? The relationship between the two families, Burn admits, remains "delicate" –controversial circumstances in which to be staging a tribute to Horse's work, so soon after his death.
To be fair to Burn, he acknowledges this. He freely admits that he has not talked to Mandy's family. "I had to deal with the question of whether I was bringing it all up again when it should be laid to rest," he says. "I'm not doing it with the intention of hurting anybody, and I'm focusing primarily on his work.
"I don't know Mandy's father at all. I just hope that they (Mandy's family] wouldn't feel upset by us doing this, but the work has a right to live on and has so much to say.
"I had reassurance from Kay (Horse's sister( that me pursuing the work would not make things worse, and she supports me. But I don't want to set myself up as a spokesman for anybody."
He will, he points out, be using the show to raise funds for a multiple sclerosis charity.
So what is Year of the Horse actually about? The text is taken directly from the commentaries Harry Horse wrote to go with his cartoons in the Sunday Herald newspaper between November 2005 and his death in January 2007. The cartoons themselves will be projected in large format, and there is a soundtrack by Keith McIvor of Glasgow club Optimo, another Horse fan.
"(In that year] he goes on a huge journey, dealing with enormous material about the Blair/Bush era and the Iraq war, and his almighty concern about technology being used as a weapon in terms of surveillance," says Burn. "There are biographical elements. He talks about the drugs Mandy is prescribed, morphine rather than cannabis, despite the fact that it is known to help MS, and about suicide.
"He quotes Schopenhauer, saying it is your choice, that suicide is only escaping from this nightmare, which I think he means in terms of their personal situation and in terms of what the world has become. I think his courage in going into such dark territory needs to be recognised."
Burn, who has starred in adaptations of novels such as Irvine Welsh's Filth and Luke Sutherland's Venus as a Boy, as well as the National Theatre of Scotland's Tutti Frutti and BBC soap River City, started out simply as an admirer of Horse's cartooning. When he died, he created an hour-long "memorial" radio show for him on arts station Resonance FM, featuring music from Horse's band Swamptrash and a reading of his best-known children's book, The Last Polar Bears, and began to use his cartoons in the regular Manifesto Politikal Kabaret nights he comperes at the Tron in Glasgow. His tribute to Horse is set to be a long-term project – he discovered just last week that he has funding to take it to this year's Edinburgh Festival Fringe.
Looking back, Burn is surprised that he and Horse never met. Both were in bands playing the underground scene in Edinburgh in the 1980s, both were highly politicised, and they shared heroes in Robert Burns and William Blake.
"I just think it was a timing thing in Edinburgh in the 1980s. By the time Swamptrash was really coming through I'd gone down to London. But I do feel an enormous affinity with him; I feel close to him, and feel that I'm contributing as much as I can to his legacy. Politically, we've got an awful lot in common. I've learned so much from him and I think it's such wonderful political art that he produced. The range of areas that he looks at, his level of satire, he's of a tradition that stretches right back, you can see Hogarth in him."
Horse's cartoons were intense, brightly coloured and impeccably drawn, full of dark humour and remorseless satire, though the driving force was usually anger.
"I identify with him on that score as well," says Burn. "It is a thing that drives me a lot of the time. You have to turn it into something creative so that it's a useful rage, rather than something that turns against you and paralyses or poisons you. It's better than shouting at the telly."
So says the man who last month led a boycott of the BBC over its refusal to broadcast an appeal by the Disasters Emergency Committee about the worsening humanitarian crisis in Gaza, vowing never to work for the corporation again: a brave step, or a foolhardy one, to take against a major employer. But Burn is not short of work. He started the year in Carrie Cracknell's Dolls, a co-production with the National Theatre of Scotland inspired by Takeshi Kitano's film, and this month appears in Andy Arnold's production of Defender of the Faith on the Tron's main stage before running up to the studio theatre to put on Year of the Horse.
He has also just won funding to tour Scotland with his band the Bum Clocks, "playing Rabbie Burns and Iggy Pop".
"That's going to be a big part of the rest of the year," he says happily. "I got a bit of a taste for it with Venus as a Boy (which opened in Orkney) but touring around with a rock'n'roll band – that's a real little heaven to me."
Year of the Horse is at the Tron theatre, Glasgow, from 19 to 28 February. Additional reporting by Andrew Eaton.
1960 Richard Horne born in Coventry.
1978 Moves to Edinburgh, sneaks into life-drawing classes at the art college. Starts calling himself Harry Horse.
1983 First children's book published. The Opopogo – My Journey With the Loch Ness Monster goes on to win the Scottish Arts Council Writer of the Year Award.
1987 Becomes political cartoonist for Scotland on Sunday, a post he holds until 1992. Is becoming known as frontman of folk-rock-bluegrass band Swamptrash.
1989 Meets Mandy when Swamptrash play a gig in Shetland. They marry the following year.
1993 Designs and writes cult computer game Drowned God for Time Warner. When it is launched in America in 1996 it sells 34,000 copies in two weeks.
1996 Publication of The Last Polar Bears, which is turned into a television Christmas special voiced by Sir Nigel Hawthorne.
1998 Children's book The Last Gold Diggers wins a Smarties Book Prize gold medal.
2004 Harry and Mandy go to live on the island of Burra, Shetland, after she is diagnosed with an aggressive form of MS.
2007 Their bodies are found at their home and police say they are not looking for anyone in connection with the deaths.