What was it about Muriel Spark, without question one of the great writers of the 20th century, that led many to assume she was difficult and unapproachable? Was it that, as one critic put it, her novels were filled with “malice and mayhem”? Or was it her uncompromising stance when dealing with those involved in what, in The Girls of Slender Means, she mischievously and mockingly described as “the world of books”? Or was it her desire for privacy, which meant she lived most of her life in exile? Or was it, perhaps, her fractious relationship with her son, whom several of her obituaries suggested she had “abandoned”?

Certainly, her personal story is complex and, at times, sad. There is more than a hint of this in her writing. But the legion of readers who love her work are attracted to it for its humour and joie de vivre; for heroines like Fleur Talbot in Loitering with Intent, who go on their way rejoicing; and for Spark’s unsentimental attitude to life. Take Memento Mori, her moving but hugely funny novel of old age. Here is the essence of Spark, brilliantly encapsulated in the refrain – “remember you must die” – which runs through its pages like the chorus in a Greek tragedy.

I first met Muriel Spark in 1990, in the Tuscan town of Arezzo. Spark was 72 and had just published Symposium, her 19th novel. By fax, her preferred mode of communication, we arranged to meet for dinner. “Daytimes are very hot,” she advised; an understatement. The month was July, and you could stir-fry on the pavement.

She arrived at the appointed hour with her companion, Penelope Jardine, an artist in her own right, in whose house – a fortress-like, 14th century former rectory in the Val di Chiana – Spark had lived for the past two decades. Spark was wearing a lemon-coloured dress and soup-plate-size glasses. She was in a flirtatious mood, chatting with the young waiter whom she described as “dishy”. I noticed she spoke good if rather formal Italian in the indelible Edinburgh accent of her most famous character, Jean Brodie.

To begin with we talked about Symposium, a novel preoccupied with madness and how those who are deemed to be mad interact with others who are seemingly sane. “Here in Scotland,” says mad Magnus Murchie, “people are more capable of perpetrating good or evil than anywhere else, I don’t know why it is.”

Spark asked what I thought of Magnus. I suggested that it was stretching credulity somewhat to have a mad man as a family mentor. But it did not seem so to her. “It’s amazing how many people do go to bins [lunatic asylums],” she said matter-of-factly, adding with a glint in her eye, “especially in Scotland.” From that moment on we relaxed and the meal passed in a blur.

A week or so after my article based on the interview appeared, I received a letter from Jardine. Both she and Spark had enjoyed it, she said. It was as if blood had been speaking to blood. Would I, she asked, as if as an afterthought, be interested in looking after their house the following summer? It was an overture to a friendship that would last until Spark’s death in 2006, at the age of 88.