Heyman, 41, actress and stand-up comic turned novelist, quickly disarms with her good-humoured Antipodean frankness. These days, she says, her life is lived between two countries on opposite sides of the world. Back home in Sydney, her accent is soft, almost British. Here, she's definitely Australian. But where is home anyway? She keeps asking herself the same question. It is, she says, one of her obsessions as a writer.
After a decade living in Scotland and England, she moved back to Australia four years ago with her husband and their two children, Seren, eight, and Taliesin, six. But they are still frequent guests at her sister's house in North London.
She feels, she says "hideously tugged, whichever side of the world I'm on. What I hadn't quite planned for was how much I would pine for Britain. I have a lot of family here, a lot of friends, in many ways the bigger chunk of my life is here. At the moment I'm coming back here every six months or so, but it's not sustainable. Financially, time-wise, family-wise, it's getting harder and harder."
A lesser woman would flinch at the thought of taking two small children halfway round the world on a regular basis, but the Heymans are already seasoned travellers. I can see her chivvying them along with that bright, can-do attitude, backed up by a readiness to laugh at misfortune. This is, after all, a woman who arrived at the Edinburgh Fringe without support, publicity or even a programme listing, and left with a hit.
You begin to see what might appeal to her about Jess, the heroine of her fourth and latest novel, Captain Starlight's Apprentice, which is out in paperback next week. It's based on a true story: Jess was sold to a circus as a child and soon makes a name for herself, riding as skilfully and fearlessly as any man. For a time her talents flourish, in the early days of the film industry, until prejudice and betrayal cut her down and force her into the life of an outlaw.
The novel, which will be serialised on Woman's Hour on Radio 4 next month, entwines Jess's story with that of Rose, an English immigrant to Australia a generation later, struggling to care for her new-born baby and hold on to her sanity in an alien land. Through this fictional character, Heyman is exploring her own mother's experience of postnatal depression - so severe that she attempted suicide and was hospitalised for six months.
"It's a bit of a mystery how a book comes about," Heyman says. "I did quite a lot of thinking about why I was engaged by the story of the lady bushranger, and I realised it was the fact that as well as being this gung-ho figure, I knew she had a child, and the child had disappeared. Once I had unpacked that, Rose's voice suddenly appeared.
"I had this sense that I wanted to explore my mother's story, but I didn't want to write a memoir, so I did it by glancing sideways."
It was only as an adult that she realised the seriousness of her mother's condition and its impact on the family. "People allude to things. I knew my mother had had a breakdown, as it is euphemistically called, like when you get stuck out in the hills with your car! In my late teens I knew she had attempted suicide, and had actually been declared dead. The bit I didn't know until recently was that I had been taken away for six months to live with an aunt. I'd always assumed I was looked after by my father and older siblings.
"When I had my own children, I had the sense of grappling with attempts to understand how she could have felt able to leave a baby. I felt that I couldn't do that, that the child would be the thing that would keep me in the land of the living. But my experience was far removed from hers.
"On the one hand, I experienced what many new parents do, a kind of forgiveness of their own parents for being found wanting. When you become a parent yourself, you realise it's actually pretty difficult. But I also had a new kind of anger, which was: 'How could you do that? What place would you need to get to to think it was a good decision to commit suicide when you had a baby?' I think in the end I did get to a place where I understood that."
Today her mother - "a ball of oomph" - plays an important part in her life. Part of the reason for moving back to Australia was to be close to her. It is only in recent years that they have been able to talk frankly about the illness.
"My mother's good at talking about these things now, she's quite open as a person and actually quite courageous. It was astonishing to me, but very matter-of-fact to her. It seems like a huge thing, but she talks about it as calmly as if it was a trip to the hairdressers."
Heyman was particularly intrigued to learn that, during the time her mother was declared dead, she experienced visions. "Sadly, not visions that have a narrative coherence of a tough-talking lady bushranger, but rocks and water. I'm very interested in that kind of near-death experience. Also, I think a little bit of me envied my mother these visions. I guess as a writer you are constantly in the act of seeking visions and attempting to straddle this world and the mythical other world. So I wanted to explore that as well."
She believes in her mother's case, as in Rose's, that the postnatal depression was created by circumstances. "In Rose's case she is removed from her family, from everything she knows, she has a shocking premature birth and is in an utterly isolated environment. What the hell do you think is going to happen? In those extreme circumstances it would take a hell of a strong psyche not to get depressed."
In the 1950s, the condition was little understood. Doctors tended either to ignore its seriousness or treat it aggressively with Electroconvulsive Therapy. In the course of researching the book, Heyman joined an online forum for those who had received ECT.
"In some extreme cases it can be effective and is still used. Some people do say it has saved their lives. But there was a period in the 1950s and 1960s when it was used too freely: 'Here's a woman who seems to be struggling with being a mother, let's give her an electric shock, that would be helpful!'"
At the heart of the book is a question about heroism. Is the gun-toting, rough-riding Jess a true hero? "I think I wanted to explore that idea of what heroism looks like. What if heroism is the action of choosing to keep going, choosing to love? Maybe in order to be a hero, a woman doesn't have to be like Odysseus, chopping off giants' heads. Maybe it's the daily changing of nappies. What if that's heroism?"
Heyman grew up in the Hunter Valley, New South Wales, the youngest of five children. Her father was a rodeo rider and horse breeder, but her parents separated when she was small. "I think there's also a sense of trying to channel him or reclaim him in the book," she says.
But Lake Macquarie was "kid heaven". "There were lots of old tin boat sheds that were falling down. Certainly by the age of ten I was out sailing in a canoe with a friend who was also ten, on our own. I'm a pretty chilled-out parent, but I can't imagine letting my children do that."
She says that "benign neglect" gave her imagination freedom to roam. "I discovered charity shops bizarrely young. I'd go and buy up boxes of books. That old book smell can still make my mouth water.
"At the time you think it's the kind of thing that all children do, and then as an adult you think: 'No, actually, for an eight-year-old to be lurking around charity shops buying old books and knitted waistcoats with their pocket money, that's actually a bit weird!' I just remember the deep pleasure and excitement I would get when I hauled those boxes home."
She was, she says, an "absolutely obsessive reader". "I can remember thinking early in my adulthood that that was a sign of family dysfunction: I was reading because I was unhappy. But now my daughter is an obsessive reader and I think my children have a fairly happy family life, so I think I can safely assume that she reads obsessively because she loves reading. Now I've had to reframe my entire childhood - I probably owe apologies to my family as well."
She also wrote obsessively. "Oh God, I do still have shelves and shelves of these journals full of jottings. I was always obsessed, even as a young child, with what lay behind people's actions. I can remember even as a primary school child watching people on buses talking and being absolutely fascinated by what their intentions were."
She now recognises that as a sign of a writer in the making, but at the time her sights were set on drama school, and she worked in theatre in Australia as an actress and playwright until the chance came to take a play on tour to Northern Ireland.
The year after, in 1993, she signed up with a company for the Edinburgh Fringe, but at the eleventh hour the company folded. With the venue still booked, Heyman struck out alone, with her self-penned one-woman show based on poetry, Dancing on the Words, and with no money and no publicity. Two reviewers on the opening night, expecting the other show, gave her four- and five-star reviews. According to The Scotsman, it was "a spirited, refreshing experience" and Heyman was "perhaps the Australian Liz Lochhead".
"That was life-changing, some kind of epiphany moment. It was to do with courage. Now, my advice to a young actor or young writer in the same position would be 'Don't be so ridiculous.' But I did do it and it did work. I think in a way it allowed me to write my first novel because I realised I could make things happen from nothing, and that's what a novelist does." The following year, for the Fringe, she co-wrote a comedy show with Jo Enwright, That's the Way to Do It, and shortly afterwards completed a Masters degree in Writing at Sheffield Hallam University. There she also met her husband-to-be, Richard Griffiths, who was training to become a methodist minister.
"I became a minister's wife. My writer and actor friends thought it was absolutely hilarious. It's quite nice now to say I'm an ex-minister's wife (her husband exchanged the church for a job in environmental planning) because people then assume I'm on to my second husband." Another peal of musical laughter.
They settled in Clydebank, then in Edinburgh, where Heyman wrote her first two novels, The Breaking, which was shortlisted for the Scottish Writer of the Year Award and longlisted for the Orange Prize, and Keep Your Hands on the Wheel. The third, written in Oxford, where the two children were born, deals with the story of the shipwreck of the Batavia in 1629 and won awards in England and Australia.
In 2002, the family decided to make Australia their permanent home. "My mother was getting older and my children had no sense of being Australian. I just wanted them to have a couple of years of being able to run around barefooted, leap in the water, all of that."
But she hadn't figured on missing Britain. She was as surprised as anyone to see that coming through in Captain Starlight's Apprentice, in Rose's aching for the bluebell woods and meadows of her English home. "My first three books were written in exile, if you like, and there is a longing in them for Australia. My agent said to me, 'What's the bet that now you get that longing for Britain?'" She grins, gives a what-do-you-know shrug. "And sure enough - she was right."
Captain Starlight's Apprentice is out in paperback on Thursday, priced 7.99. It will be serialised on Woman's Hour on Radio 4, 2-6 April.
The truth about postnatal depression
Gail Porter, TV presenter
"I was crying a lot. It was about a year-and-a-half on and I still kept thinking I was doing this and that wrong and I wasn't a good mother. I went to a doctor and could not even speak to him. I was just crying. He prescribed me Prozac but I felt guilty about that as well, although when I started taking it I felt better. Then I just came off it without telling my doctor. That's when I hit rock bottom."
Melinda Messenger, TV presenter
"Emotions are very powerful, but because they aren't physical you don't always want to admit to them. I think it is important that women realise that post-or prenatal depression is not a weakness."
Natasha Hamilton, former member of Atomic Kitten
"My doctor diagnosed me with postnatal depression and ordered me to take time off work. I wondered why I was feeling so down; I was crying a lot and everything got on top of me. I was surprised when the doctor told me, but glad to know I wasn't losing the plot. I couldn't understand why I was losing weight, but it's common with depression. Now I will take care of myself and get better."
Courteney Cox Arquette, actress
"I went through a really hard time - not right after the baby, but when [Coco] turned six months. I could not sleep. My heart was racing. And I got really depressed. I went to the doctor and found out my hormones had been pummelled."
Brooke Shields, actress
"In a strange way, it was comforting to me when my obstetrician told me that my feelings of extreme despair and my suicidal thoughts were directly tied to a biochemical shift in my body. Once we admit that postpartum [postnatal depression] is a serious medical condition, then the treatment becomes more available and socially acceptable. With a doctor's care, I have since tapered off the medication, but without it, I would not have become the loving parent I am today."