They met at a Roman hairdresser's shop. Penelope Jardine was 36, an orphan since her mid-teens and a student of sculpture at the Accademia di Belli Arti. Muriel Spark was the famous Scottish author of The Prime of Miss Jean Brodie, The Girls of Slender Means and seven other novels. She was 50 and "quite menopausal" at the time, recalls Jardine, and being driven up the wall by her American secretary who "could speak American but not spell too much English".

Having recently bought a car, Jardine was short of cash and on the way to lunch with a friend. When the friend spotted the famous writer having her hair done, she persuaded Jardine to pop in and present herself as a possible replacement. She left a card, and "about six months later I had a telephone call in the middle of a party I was giving and heard this voice saying 'Are you Miss Jordan?' And I said 'No, I'm Jardine actually.'"

Muriel Spark asked if she could type, take shorthand and speak Italian. When Penelope answered yes to all three questions ("I semi-lied because I wasn't very good at Italian at that point") she was offered a job.

Jardine tells this story in the dimly lit sitting-room of the house she shared with Spark in the Tuscan countryside for more than 30 years. When Spark died in Italy in 2006, Jardine became her heir and literary executor. Having recently finished work on her first book, an edited selection of Spark's essays, and disappointed by the way her friend has been portrayed since her death by biographers, she agreed to be interviewed at home about their life together.

Jardine's first assignment was to organise Spark's library: "She'd just come from America and had all these books she wanted on the shelves in a certain order. She said just put them up – history, belles-lettres, novels, biography. And I thought, what exactly is belles-lettres? Half the books I'd never seen before so I had almost to read them before I could place them."

It was several years before the women moved in together and Jardine says she can't say exactly when their relationship switched from a professional one to the personal one that shaped her life. Over the years there were inevitable hints that the women were romantically linked but Jardine, 82, says the reality was simple: "We were very good friends, we really were. That's what it was, nothing else."

They never said to each other, let's make a life together, but time went by and there they were, "still doing the same thing" – working at home, taking their meals at local restaurants and going on long driving holidays across Europe because Jardine didn't like flying.

When they met in 1968, Spark had been divorced for 25 years and had a dramatic early career in publishing behind her, along with several relationships. She had also converted to Catholicism, which became the meat of a lifelong quarrel with her son Robin, a practising Jew who insisted, in the teeth of Spark's denials, that her mother as well as her father had been Jewish.

She moved to Rome from New York, where she had worked for a time in an office at the New Yorker. To start with, Jardine lived down the road, but in 1973 set off to buy a house in Tuscany from a list obtained by a friend of ruined church properties for sale. Spark followed her to Oliveto, near Arezzo, a year or two later, drawn by the promise of cooler air in which to finish her 14th novel, The Takeover. "She used to say she never moved out," recalls Jardine, "but that wasn't quite true because she didn't like it in the winter".

In the preface to the new essay collection, published in the UK as The Golden Fleece (and in the US as The Informed Air), Jardine is critical of Spark's biographers, saying none of them has so far picked up her "brave and generous spirit nor the optimism and joy of her personality". Reviewers disagreed. Martin Stannard's Muriel Spark: The Biography was widely praised when it came out five years ago, not least for the tactful way it handled the writer's estrangement from her son.

But perhaps those closest to a person are least likely to be satisfied by an account of their life produced by an outsider. With its dramatic failures of marriage and motherhood, Spark's will never be an easy story to tell. Born Muriel Camberg in Edinburgh, in 1918, she used to joke that her husband's name was the best thing about him. When she left him and their son, Robin, in southern Africa in 1944, returning to Britain alone on a troopship, the boy was just five years old.

He followed her the next year and was sent to live with his grandparents in Edinburgh. Mother and son kept in touch but a pattern of estrangement and blame set in. Spark's death was followed by lurid reports of a letter she had left explicitly barring Robin from any claim on her estate. Robin, an artist, responded by saying he didn't want an inheritance anyway: "You only need one bed to sleep in, one house to live in. I'm lucky. I've never put great value on money per se." Muriel had no grandchildren.

Jardine agrees this part of Spark's life makes a sad story and thinks there is a book to be written about famous women falling out with their sons. "I think in the early days she'd have liked to have some man she liked enough to stay with, and she definitely wanted a father for Robin," she says. "She looked for a father figure and never found a suitable one. She looked in the wrong places. She said herself she picked very badly and she did. The men I knew her with were not suitable at all."

Did Jardine also hope to meet someone? "Yes I did, but they didn't come along. Muriel remained quite keen to know everybody and maybe to meet another interesting man. But there were too many homosexuals around, for one thing, in the art world."

So they kept each other company, going on fabulous road trips to Venice and France, Germany, Holland, Switzerland and Spain, and entertaining visitors. Jardine still drives the old BMW in which Spark used to keep whisky and cognac miniatures in the side pocket. The former Guardian columnist Alexander Chancellor was a neighbour, who got them involved in a local campaign when hunters poisoned several dogs. There were also repeated burglaries, in which Jardine lost many of her own paintings and sculptures.

What of her own life as an artist? A few pictures remain on the walls of her home. How seriously did she take herself? "Oh, I think I took it very seriously. I gave it up just because the time wasn't there. She sort of swallowed me up." Is this a regret? "Sometimes, but you can't have everything, can you? You can't do everything. It happened slowly. To start with I was just secretarial, doing the letters, then I'd be doing the filing and I'd know where things were. One way or another, you just find yourself doing it."

Jardine is sure Spark saw what was going on, but "didn't say anything in case it changed". She laughs. "I suppose it suited her because I could drive her and I could cook a bit and I had a house and all those things."

For all the help Jardine provided to Spark, it was Penelope's house that they lived in and her decision to move to Tuscany. Both women found themselves without conventional family ties – Jardine had lost both parents by the age of 14 – and in each other they found a solution.

"It's a very strange thing being creative. I don't know if I can boast about it but I do know how lonely it is," Jardine says. "I think she found that and I think the fact that I was there with that sort of feeling too was a good thing for her. It was a good thing for me to have someone around in this huge house to say 'What have you been doing?'

"Otherwise you're just in your studio painting away, or writing away, and at the end of the day you're worn out and there's nothing to lift you up again. Unless you have a good man, which would be ideal, of course, but it doesn't always work out."

Today Jardine's work as Spark's executor keeps her busy most days with requests for permissions and, recently, the essays. She is thinking about a book of aphorisms, for which Spark characters such as Mrs Hawkins and Miss Jean Brodie are famous.

She likes the novels very much. "There's no bad book. They were all good. The ones about London I think are good and I love the early stories about Africa. I think they're wonderful and don't know why they're not collected together."

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But she gets sick of people going "on and on and on about Brodie", the teacher with fascist sympathies from the novel that Spark based on her own years at James Gillespie's High School for Girls in Edinburgh: "Muriel used to say it was her milch cow, like Tess of the D'Urbervilles was to Hardy."

Spark's only volume of memoir breaks off in 1957 and so there is no account in her voice of her long years spent with Penelope Jardine. Stannard wrote of the friendship as Spark "learning to love again", but Jardine thinks this is a bit soppy. When Spark "occasionally stuck the dagger in" it was devastating, but mostly she enjoyed her friend's company and found her less difficult than others did: "She was a very complicated person in some ways, and very simple in others. She was quite easy to get along with, very easy to talk to. She wasn't a snob and she wasn't grand about what she did. In fact, you usually felt rather good in her presence. You felt you could do anything."

If some sadness surrounds Penelope Jardine's surrender of her own creative ambitions as well as her lack of a life partner and family, she is proud to have played a key role in one of the 20th century's great literary careers. "I think I was born to play second fiddle," she says, "not the first violin, the second violin."

Jardine saw Spark as no one else did: "It would all be in her head for about a year and then suddenly she would what she called 'strike', and she'd be writing very fast."

While she waited, Spark would mooch about their big house and "often make her face up. She loved playing with disguise, putting on lipstick or combing her hair or doing it in a different way. It was vanity, yes it was, but it was also a game I think. I was always walking into her room when she was putting on her eyebrows and that would annoy her greatly. Her eyesight wasn't terribly good – perhaps she thought the eyebrow wasn't quite straight and then I'd walk in saying something." Without her, she says, the house feels much emptier.