Putting real historical characters into a novel is a minefield, especially those who existed within living memory. How much more so when it’s your own family? My relationship to my grandfather is complex and constantly evolving. I’m conscious that his story belongs to different people in different ways, within our family and also more widely. I think part of the appeal of writing this book was trying to reconcile the ways in which I’ve come to understand him: as a grandfather; a father; a friend; a traitor; an idealist. But I had to find the right way to approach it. When I happened upon the story of Edith Tudor-Hart, I knew that she was the person I had to write about. She’s always cast as a bit player if she’s mentioned at all, but she was a remarkable woman. Anthony Blunt referred to her as “the grandmother of the Cambridge spies”.

Why do you think she has been overlooked? Partly because her files were only released to the National Archives in 2015. But more generally women’s roles in espionage have been sidelined, and I wonder if that’s because a lot of this history has been recorded by men. When you look at the way Edith is referred to in the [security service’s] files as “that foreign woman”, I wonder whether there’s a sense of embarrassment too. It was bad enough with Kim and the Cambridge spies showing up the British establishment, but the idea that this foreign Jewish woman might run rings around men in positions of authority, perhaps that was embarrassing for them.

She’s a sympathetic character in the novel. Did you feel that as you uncovered her story? I feel a huge amount of sympathy for her situation, and admiration, really. And sadness. She was incredibly brave, incredibly loyal; she was completely faithful to the things she loved and believed in, and she had a firm belief that she had to do the right thing. Regardless of what I think of her ideology, I greatly admire the fact that she was able to hold these various parts of her life – being a photographer, being a single mother – alongside being a revolutionary, feeling that she could change the world.

Were you always aware that your family had this extraordinary history? Did your father, John, talk about his father? We never had a family meeting where he said: “Right, I’m going to explain to you who your grandfather was.” That was not at all the kind of person he was. But we visited Kim in Moscow and those holidays form some of my earliest memories, so it wasn’t like he was a secret. He was my grandfather, and then as I grew older it became apparent that he was someone else, too. Recently I found a paparazzo shot of my dad with a suitcase getting into a car and me, aged five, looking into the camera, and I had this moment of recognition. That was my dad leaving to go to Kim’s funeral [in 1988] and we were hounded by paparazzi. I remember at the time feeling that something very dark was happening and that I didn’t feel safe.

How easy did you find it to capture Kim’s voice? I spent so long with his letters and they gave me such an insight into the expressions he used, the way he spoke, how he flipped between being tender and reflective and witty and scathing, and his very English obsession with the weather. I love those images of him as a real person; for me, that’s where the clues as to who he really was can be found. A large amount of the letters in the book are lifted verbatim from those.

You’ve written three previous novels about espionage and betrayal – did you need to approach that world obliquely before tackling Kim’s story? I was adamant that I didn’t want to write about Kim because ironically I didn’t want to define myself through his life. But they were quasi-spy novels so I ended up talking about them in relation to his story anyway. I think I’ve spent so long having other people project their versions of my grandfather – and my family – on to me that writing about it is a way of trying to reclaim the story. There are so many ways that Kim’s choices have continued to occupy my mind since my dad died in 2009, but I do feel that this book draws a line under it. I feel as if I have been through some form of therapy, I’ve purged these questions in a way.

Where do you write? I wrote Edith and Kim all over the place because we’ve been in the process of moving house for the past couple of years, but now I have a shed at the bottom of our garden where I can feel I’m in the middle of nowhere, and I’m never leaving.

What are you writing next? After publishing four novels in four years, I’m taking more time to write the next one. Writing Edith and Kim alongside home schooling and everything else has been a big thing and I feel now is a good time to pause and reflect. I’ve recorded a podcast with the BBC that will be announced later this year, and my husband and I have bought an old pub that we want to turn into a creative space to make available to charities and people who wouldn’t otherwise have access to that, so that’s an ongoing project.

What are you currently reading? I’m trying to reconnect with what made me fall in love with books and the process of writing so I’m going back to reread some of the books that feel seminal for me: The Beach, The Child in Time, White Teeth, Disgrace, The Poisonwood Bible and The People’s Act of Love.

Which author do you most admire? I don’t have favourites, but I love Helen Dunmore. Exposure showed me – along with William Boyd’s Restless – how you can write a novel that is ostensibly a spy story but is really about the people, and the families, at the heart of a great betrayal.

• Edith and Kim by Charlotte Philby is published by HarperCollins (£14.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply