What is left behind by a life? Avoiding this question can be as much of an art as answering it. Philip Larkin wrote of ambulances, inexorable as plague carts, extracting those who are about to die from the commerce of the living. In his poem “Aubade” he describes the blank horror of extinction, the mind caught in the glare of it, the poet half-drunk and paralysed by dread of death. In this poem the living leave no legacy, unless it is self-delusion.

I have been thinking about this question of legacy over the past few months, for one reason because my new novel deals with memory, historical record, what remains, what is saved and what is lost. The question has become more acute because a few months ago I was diagnosed with a cancer that has a very poor prognosis. The ground beneath my feet has never been more uncertain, but what is sure is that the ambulance has already called and there is no vagueness about my mortality.

There never was, although I might have fooled myself about it. A couple of generations ago I’d have done very well to live in reasonable health to the age I now am: 64. Go back another generation or so and childbirth, monstrous figure of attrition, cut short the lives of innumerable women, as it would have ended mine. Men’s lives were scrubbed out by the first world war, children were killed by diphtheria, whooping cough and minor infections that bloomed into sepsis in the days before antibiotics. Death and the living walked hand in hand and could not easily pretend that they had nothing to do with each other. Inscriptions on gravestones acknowledge how soon the living will join those who are already under the earth, not lost but gone before. The grief of death is admitted but there’s no incredulity, no sense of thwarted entitlement.

But in western Europe in 2017 there is a narrative of lengthening lifespans, of extraordinary treatments that fend off death for decades and may, in the end, outwit it entirely. It isn’t realistic and in our hearts we know this, but there is a tendency to think and talk as if it is not the mark we leave upon time that we need to think about but the endless years that will be ours if we eat well enough and exercise effectively enough to dodge the grave.

In much of the rest of the world this magical thinking won’t work. Infant mortality is appalling, water kills and so do far too many curable diseases. Dictators herd their opponents into cellars before hanging them in batches. I may be ill but I’m also warm and sheltered, surrounded by family and friends and with medical help a phone call away. I think of a young man or woman in the Middle East who has lived less than a third of the years that I’ve enjoyed and is now alone in a cell, tortured, condemned to death, silenced and very likely denied even a funeral. More than ever it seems a heartrending abomination that human beings, who can’t avoid natural suffering and death, should inflict further pain upon one another.

Most of us die in silence and leave silence behind us. There is no visible mark, no written record and very often no grave to visit. Ancestors have shifted about in search of work, or been unable to write, or have never had the cash to pay a stonemason. They leave behind a story, perhaps, or an anecdote that is handed down from child to grandchildren, and then is heard no more. Existence subsides into a humus that at first sight looks entirely anonymous. But I want to probe more deeply, because I believe that there is more to it than that. Anonymity is also an inheritance and perhaps a precious one, just as the poems grouped under Anonymous in an anthology are often the most moving of all, honed as they are by generations of memory.

In my new novel, Birdcage Walk, I write about a small group of radicals in Bristol at the end of the 18th century. It is the time of the French Revolution, when “bliss was it in that dawn to be alive”, when idealism and curiosity blazed through a thousand speeches, pamphlets and poems, when speakers toured mills and factories and women wrote about that impossible but almost imaginable future when human rights might be for human beings rather than for men only, and when those doubly silenced might be allowed a voice.

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The key thing about fiction, as in life, is that no character knows what is going to happen. Every one is locked into the present moment, as we are now. They do not expect the revolutionary wars between Britain and France, or the collapse of the building boom in cities such as Bristol that war will cause. They don’t know that private ruin will be part of public upheaval, that dozens will go bankrupt, that labourers will be laid off, and ruined and disappointed speculators will take any risk to get their money back. They look to Paris and see ferment working at the body social and political like yeast. The guillotine has not yet been trundled out to industrialise state murder, because argument and dissent are still considered a valuable part of the revolutionary consciousness. But all these things will come, and with them a momentum that crushes the individual conscience.

Birdcage Walk really exists. It’s a path leading through the graveyard of a church in Clifton, Bristol, which was bombed in the second world war. No fresh graves are dug there, and everything is overgrown with bramble, goldenrod, periwinkle, rosebay willowherb. Overhead grow pleached lime trees on an iron frame. At the opening of my novel, a man walks through this graveyard with a dog, which scratches at a worn stone dating back to 1793. It marks the grave of a woman, a pamphleteer and political activist. Her name is Julia Fawkes and when this man searches for her online he finds no trace. I wanted to write about a character who was passionately admired, much criticised, loathed for her unfeminine insistence on the importance of rights and money to women, but whose legacy did not survive. Or you might view it differently, and say that the influence of women like this, while nameless, soaked into the fabric of their times and changed their colour.

Bristol’s history of radicalism is rich with pamphleteers, philosophers, poets and advocates of political change. Some of them are known and named; many are not. They were watched, followed, threatened, repressed. I did not use these people as models for my characters, but in their intensity, boldness and occasional absurdity they inspired me. I wanted to write about people who had not left behind their own accounts of their lives. Their letters have been lost, their pamphlets have not been preserved, their money has not amounted to an inheritance and they had no epitaphs. Historical record does not know them but fiction can imagine them.

Even if nothing concrete has been handed down, that isn’t to say that an individual has left no trace of her existence. A look, a gesture transmitted from parent to child, a way of dressing, a song sung by a bedside may travel down the generations. It’s an intangible legacy but a very real one. Perhaps a child of 2017 might stand next to her great-great-great-grandmother and reveal their kinship in a turn of phrase. We die, but do not quite die out.

In my novel, Fawkes’ daughter separates herself from the radical political traditions of her upbringing when she marries a property developer. Property was then, as it is now, a shibboleth. Everybody wants to pile in money because the rewards look certain. I wanted to write about the strength and vision of the developers who devised those iconic terraces of Clifton, but also about greed, self-delusion and the psychology that creates a property investment bubble. Then, as now, it seemed that money put into bricks and mortar could not fail to breed. These magnificent terraces slung 200ft above the Avon Gorge were bold in their engineering and brazen in their expectation that money would draw money. But within a few months the vision failed. For years the terraces of Clifton would remain half-built. Contemporary visitors said that this urban landscape of roofless terraces looked like the ruins of a city after war or plague had torn through it. Dozens of builders and developers went bankrupt. What had seemed certain was revealed as utterly insecure.

Those terraces are still here, more than 200 years later. They have gone through many incarnations: at times almost derelict, they have been warrens of bedsits, or damp and leaky flats. They have attracted conservation grants and then the rich have flowed back into possession. The honey-coloured stone is cleaned and the broad private pavements covered with restored flagstones. The past, once again, survives into the present.

Birdcage Walk is overgrown at the height of summer, and full of shadows. The graves lean and stone angels gaze over a sea of weeds. There are dog walkers and sometimes you’ll see a figure doing tai chi in a clearing: this is Bristol, after all. I have walked through this graveyard for more than 40 years now but it is still mysterious to me. No one puts flowers on these graves, because the dead who lie here died too long ago. What is left behind by these lives? I think of Larkin’s horror of extinction, but this is not a place to horrify. The history here rises from below and must be listened for, because it hasn’t got a place in the canon.

I write fiction, not history. If I’m lucky, someone will stop to read it and then walk on, imagining other times and other lives, so far away and yet as near as the brush of leaves over that stone, there, where the snuffling jack russell has uncovered an inscription.

• Birdcage Walk by Helen Dunmore is published by Hutchinson. To order a copy for £16.14 (RRP £18.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.