A young mother ambling across Wimbledon Common with her dog and her young son on a perfect July morning in 1992. A frenzied attack from an unknown assailant. A media circus. A botched police investigation.

The murder of Rachel Nickell was one of the most high-profile crimes of the last few decades. Her name conjures an instant picture of her blond hair and laughing face. The names and faces of her young son – who witnessed the attack – and her bereft partner, Alex and André Hanscombe, are probably less familiar, which is just as they wanted it. Unable to live in the spotlight, they left the UK just months later to begin again. Now, 25 years later, side by side in a hotel bar in sunny Barcelona, smiling, relaxed, this tight father and son team seems a kind of miracle.

Alex, 27, has written his own account of his mother’s murder, its impact and aftermath. It’s as he remembers it – through the eyes of a child. Though he was just three weeks short of his third birthday when it happened, he was able to provide police with a strikingly accurate account of the attack and the attacker, and it remains crystal clear in his mind.

Rachel Nickell and Alex. Photograph: André Hanscombe

At that time, Rachel Nickell, 23, lived in Balham, south London with André, Alex and their rescue dog, Molly – she called them her “little pack”. She had met André at a swimming pool when she was just 19, lifeguarding. A former grammar-school girl, Rachel was studying English literature at university; André was a semi-professional tennis player. After their first date, Rachel phoned her mum to say she had found the man of her life. Within months, she was pregnant, had quit her studies and moved into André’s flat. He’d started work as a motorcycle courier to make ends meet. The plan was to sell up, leave the UK and live somewhere rural – France was top of the list.

“I remember waking up on the morning it happened, waving goodbye to my father, watching him fire up his bike for work,” says Alex. “The journey across town to the common has kind of disappeared, but I remember walking along the path in the woodland.”

Suddenly, a man appeared from nowhere. Alex later described him to police: his loping walk, his strange, blank face, his black bag, white shirt. He threw Alex to the ground – and by the time he had recovered himself, the man was down at the stream, washing his hands before lurching away. Rachel – who had been stabbed more than 40 times – lay utterly still.

“It’s not like it happens in the movies,” says Alex. “It was so quick, and everything was silent. There was this strange polarity – even though it was hectic and violent and there was blood, at the same time, there was this big feeling of peace and tranquillity. To me, my mother just looked like she was lying there, ready to wake any moment, like in the imaginary games we used to play.”

Alex asked her repeatedly to get up – but Rachel didn’t respond. “That’s what I remember most,” says Alex, “the particular moment I knew she was gone. That feeling of losing someone you love, how everything can change in a matter of seconds.”

Rachel and André with baby Alex. Photograph: André Hanscombe

It seems extraordinary that Alex never asked for Rachel again, or wanted an explanation for where she was or what had happened. In his little world, he’d already formed an understanding of death. The family had recently rescued a bird with a broken wing, which thrived for a while then died. They had buried it on Wimbledon Common and Rachel had explained he “didn’t need his body any more”, the part that was “really him” had “gone somewhere else”. Alex had also been transfixed by the children’s film The Land Before Time just days before the attack, where the mummy dinosaur dies in an earthquake and the baby pleads for her to get up, before carrying on without her.

On the common, the alarm was raised, an ambulance called, and Alex sedated. From the moment André collected him from hospital, Alex transferred all his needs, his love and emotion to his father. When Alex woke next morning, he asked immediately for “Daddy” – until that day, it had been “Mummy” every time. In the following weeks, Alex wouldn’t let André out of his sight.

It’s impossible to imagine how André functioned, raw with shock, deep in grief, the focus of a media frenzy and sole protector of a traumatised child – who was also the only witness to the murder, the best possible source for leads.

The fact that he’d switched all his passion and attention to me was overwhelming but also a massive aid

“It was mind-bending the whole time,” says André, “and there wasn’t anybody I could go to for help because it was one of those extreme experiences that very few people had encountered.”

Rachel was his reference point. “We’d had a fairly normal dynamic,” he says. “I went to work, Rachel was home with Alex. The two were a unit, a circle, and I was on the outside, controlling the perimeter. I didn’t take in all the details of Alex’s day because it wasn’t my responsibility. But ‘normality’ for a three-year-old means getting the toast right, the showers at the right time, the juice in the right cup. For every decision I made, from the most unimaginably trivial, I’d ask myself: ‘What would Rachel have wanted?’ And because I knew her so well, because we had that connection, that’s what I leant on.”

That – and Alex himself. “The fact that he’d switched all his passion and attention to me was overwhelming but also a massive aid,” says André. “It was soothing, a balm. That love and affection keeps you afloat.”

From the outside, the decision to leave five months later, just the two of them and Molly the dog, seems extreme. Their first stop was the south of France, where they rented a remote farm building near Montpellier. Three years later, they crossed over to Spain, settling in Catalan countryside, where André coached tennis. They told no one about their past.

Starting over was, says André, a nightmare. But the alternative was worse. Normal life in the UK had seemed impossible, unreachable. The press had followed Alex to nursery. The police had wired their home in case Alex revealed a crucial detail. They had been taken to gruelling sessions with a child psychologist, which felt all wrong.

“All the time, you see the headlines, you hear the whispers, everyone’s watching Alex, saying he’ll never recover,” says André. “I just felt it was impossible for a child to grow up like that. Rachel and I were always going to leave the country – again, it was being true to her values.”

Rachel’s parents were devastated by their departure, describing it as a second bereavement. However, their relationship with Alex continued, with Alex visiting until the age of eight, when contact ceased. (They have corresponded more recently.) According to André, this was never intended – Rachel’s parents had already lost enough, he says. “For years,” says André, “life was a fog.” He battled “waves of grief”, and was often absorbed by the murder and all he had lost. “There were times when all I thought about was revenge,” he says.

The police had quickly identified Colin Stagg as the attacker, a loner who fitted Alex’s description. With no evidence, their attempt to “trick” Stagg into a confession using an undercover female police officer as bait was thrown out of court. Yet he remained the only suspect.

His nightmares had started very shortly after the attack and went on for years

Alex never asked about the case, or what had happened to the “bad man”, but Rachel’s presence remained close. “She was a natural part of the day,” says André. “Alex might draw a picture and say, ‘Mummy would have liked that.’ He had the scent she wore in his drawer with his favourite things and he’d get that out. I could see him connecting with the memories, getting comfort from that.”

By day, Alex seemed to be recovering well. “He had a huge amount of freedom,” says André. “He ran wild with the bikes and the dog, he quickly made friends, he was sociable, no signs of fear.” But nights told another story. “His nightmares had started very shortly after the attack and went on for years. He’d make these terrible, alien sounds of distress, he was catatonic, his eyes were open and I couldn’t reach him. The relief, over time, was seeing them dropping in intensity, becoming more spread out. There was improvement. There was hope.”

Neither had therapy; no one supported them. André thought of turning to a local priest but in the end, he realised day by day, just getting by, living life was their recovery. Inevitably, there were tensions.

“All the pressures on my father, having to make so many decisions, being isolated – sooner or later, the tensions were directed at each other,” says Alex. “We had a lot of fights and that really reached boiling point in my teenage years.” André agrees: “We were at war. Two males. Big personalities. Me setting boundaries and him trying to break them.”

It was only after Alex left home to study music in London that their relationship recovered. André recalls the turning point. “Teenage kids phone home for a reason – to send money,” he says. “We had the usual conversation about resources but then Alex didn’t end the call, he wasn’t running away to do something else. He started asking me: ‘So, how are you?’ He was sensitive, there was so much emotional intelligence. I put the phone down and thought: ‘I’m dealing with a grownup.’”

Rachel’s killer – Robert Napper – was not identified until 2004. By then, he was already living in Broadmoor high security psychiatric hospital after committing more than 100 sexual offences and, in November 1993, brutally murdering another mother, Samantha Bisset, and her four-year-old daughter, Jazmine. In December 2008, he pleaded guilty to Rachel’s murder. Alex and André say they feel no anger towards him – nor towards the police for their catastrophic focus on the wrong man, Colin Stagg.

“I’d been very angry for a very long time,” says André. “But reading the psychiatric report and seeing what Robert Napper had been through, he’d had a violent home-life, he’d been sexually abused. I’d always told Alex that nobody’s better than anybody else. You have to try to understand why they do things. Forgiveness is a process and over the years, almost without knowing it, I’d been through mine and Alex had been through his.”

André also wrote a letter of apology to Colin Stagg for publicly pointing the finger towards him. “As a parent, you have to set an example,” he says. “I can’t expect Alex to behave in a certain way if I wasn’t prepared to do it myself. I made a mistake – based on information from apparently reliable sources. I helped put a spotlight on him, which made his life worse.”

My message is: ‘There’s light at the end of the tunnel.’

A few years later, Alex and André decided to go travelling together. “It was a completely different dynamic by then, both adults, no obligation,” says André. “We closed everything down, gave away what we couldn’t sell, then spent four years on the road – India, Egypt, Thailand, Indonesia, Sri Lanka …”

Alex’s book began along the way. “When we told people the story, it was amazing how moved people were,” says Alex. “People from completely different walks of life who knew nothing about us were inspired. I was the ‘tragic tot’ but we’d both got through. Everyone has challenges and obstacles and fears. My message is: ‘There’s light at the end of the tunnel.’”

Though there have been other relationships, André is single and he and Alex now live in Barcelona. He sees Rachel in his son every day. “Her sharpness of intelligence, her wicked sense of fun and her movements, too. Rachel was tall and elegant – a dancer. Alex has that same elegance, you see it in his gestures, his eyes, his presence.

“Nobody does everything right – and I don’t know how I did anything, looking back on it,” André continues. “It’s been a marathon. But I can look at Alex now and think: ‘I’m in clear water.’ Mission accomplished. Alex got me through – and Rachel.”

• Letting Go: A True Story of Murder, Loss and Survival by Alex Hanscombe (HarperCollins, £7.99). To order a copy for £6.79, go to bookshop.theguardian.com or call the Guardian Bookshop on 0330 333 6846.