Kerry Needham is composed and glamorous, the worldly and articulate author of an autobiography out next week, almost unrecognisable, in fact, from the broken, teenage waif who first hit the news when her little boy Ben went missing on the Greek island of Kos in July 1991. The girl on the TV back then sat helpless and child-like between Eddie and Chris, her grief-stricken, guilt-ridden parents.

Chris and Eddie Needham and their sons Stephen, aged 17, and Danny, 11, had gone to Kos, abandoning their native Sheffield, for a future in the sun, and were living in a caravan in a place called, ironically, Paradisi. Eddie, a builder, found work renovating an old farmhouse on a remote hillside track above Kos town. Stephen was helping him. Kerry, then 19, and Ben, aged 18 months, joined them a few months later. She had been lonely in Sheffield without her family and unhappy in her relationship with Ben's father, Simon. On Kos she got a job at a hotel serving at the bar and round the pool, and she had a flat of her own. Suddenly she had it all – a job she enjoyed, a life in the sun with her beloved Ben and her family. She was in her element. She said recently that perhaps they'd have lived there happily ever after if it hadn't all gone so horribly wrong.

On 24 July 1991, Eddie, Chris, Ben, Stephen and Danny were at the farmhouse having lunch. Kerry was at work. Ben was running in and out, playing with water, amusing them. After lunch Stephen went outside and got on his 50cc motorbike to leave. Ben wanted a ride. "No," Stephen said, "go back inside." Stephen drove off. He was the last to see Ben.

A few minutes later Chris realised Ben had gone silent. They looked everywhere – up and down the track, across the fields. Nothing. No sign of him. He had vanished. All around was silent, apart from the wind and the creaking of the cicadas in the overgrown fields.

In the years since then the story has been in and out of the news, flaring up when there is a particularly striking "sighting" and compounded by Madeleine McCann's disappearance in 2007, when Kerry once again found herself the object of intense media interest.

The fear of losing a child is familiar to every parent. The year after Ben vanished, I was in Crete with my partner and our two-year-old son. Haunted by the story of Ben, I didn't let my son out of my sight. One day I saw a little blond boy playing outside a shuttered villa wearing a T-shirt with Kos written on it. My suspicions aroused, I took some photos.

On my return home, I took the photos to the Needhams at their home on a Sheffield estate. It was easy enough to find them – there were no gatekeepers or media go-betweens. I got their number from the phone book and rang them. They were living on one of the old Sheffield housing estates of red brick houses and generous gardens high above the city. Eddie talked but Kerry, bleached out, aspen thin, a hovering, agonised presence, didn't say a word. Chris stayed in the kitchen. She later told me she took refuge in domesticity to ward off the pain. My snaps offered no hope but drew me into a story and a family with which I am still involved 20 years on. Soon after, I wrote the first of a number of stories about them, for the Guardian.

Back then, Eddie was obsessive, continually reassessing every possible explanation. They have always thought that Ben was kidnapped, destined for the child-selling rings that Greek police have cracked, involving doctors, lawyers, Gypsies and social workers. In those early years Eddie kept the search going, following up every lead he could, despite having no money. There have been hundreds of sightings over the years and Eddie, sometimes with Chris, occasionally with Kerry, would go to check them out, sometimes backed by a newspaper or TV news team, other times on his own, sleeping rough if he had to. They raised funds selling bric-a-brac at car boot sales and rattling buckets outside rock concerts.

Over the years I have written several pieces about the Needhams, as well as being involved in TV and radio coverage of the case. The family were always grateful for publicity, as it kept the search alive. The media have been their greatest allies. A working-class family, the sympathy for them in the tabloids made Kerry and Ben household names. They have never got much help from the UK authorities until relatively recently, and none at the start, in the days immediately after Ben vanished. When, in despair, they asked to be repatriated two months after Ben disappeared, the UK authorities in Greece said they would have to be means tested, before paying up. They were so broke they sold Danny's toys to make ends meet.

In November 1996 I was associate producer on a Channel 4 documentary telling the story so far, and following new leads. We went back to Kos with Chris and Eddie to the farmhouse, reliving the moment Ben vanished. Our story followed the dramatic twists and turns in the investigation. We shot some of it in the town of Veria, near Thessaloniki on the northern Greek mainland. I got a sense of the pressures involved in their search when one night in Veria a man appeared from the shadows looking over his shoulder, saying he had seen Ben in the clutches of a Gypsy of whom he was so terrified that he didn't want anyone to know he had spoken to us.

I saw Kerry in December 1996. She hadn't come to Greece to make the film but agreed to be interviewed in Sheffield. She had increasingly retreated from the media glare. It was evident at a glance when she arrived at her parents' house that something was very wrong. Her daughter Leighanna, born in February 1994, was living with Chris and Eddie. The change in Kerry was astonishing, the atmosphere tense, a vivid anger coming off her like static. The dress she planned to wear for the film was disco chic – a minimal stretch of grey PVC with a jagged hem. Her mother insisted Kerry wear a frock. Kerry yelled at her: "You want me to look like little fucking Mary on the little fucking prairie."

The relationship between Kerry and her parents was in freefall and they were barely exchanging civilities. Kerry was in search of oblivion, working in a bar, her nights a blaze of amphetamine and ecstasy, taking refuge from states of mind few can fully imagine. I sympathised with Kerry but understood her parents' fury and alarm. Kerry couldn't articulate what was happening to her then. She told me three years ago what she had been going through at the time. When Leighanna reached the age Ben was when he vanished, Kerry felt at breaking point. Something had to give. The teenage single mother, bereft at 19 in a way too shocking to comprehend, had faced hope then despair over and over again for almost five years, with every supposed sighting and snapshot of little boys people thought might be Ben. One tourist even sent a strand of hair from a child's head, so convinced was he that this was Ben. Kerry sometimes followed up leads but she was too frail to cope. She cites the time she went to Bodrum in Turkey after a tourist reported seeing a blond baby boy who looked like Ben in a market. That turned out to be a girl. Someone else reported seeing a blond boy in a taverna in Greece with dark-haired parents. When the child appeared, he looked so like Ben that her heart missed a beat. "Every time it wasn't Ben it was like losing him again," she told me.

There were times when life felt impossible. "I wanted to end the pain," she says. "I thought nobody else could understand what I was going through. I stayed in bed day after day. The years went on and I thought, 'I'm never going to find him, so I might as well end it.' Without Ben I didn't want to live. I cut my wrists twice and took pills twice. I'd sat at home for four and a half years crying… I wanted out."

Having survived all this, she emerged feeling that nothing more could hurt her. "I had no emotions left and I decided to go out and enjoy myself." Cutting herself off from her family and daughter meant that she could opt out for a while. "In the bar I worked in, I was just a young woman out to enjoy herself. After work I'd go dancing with my workmates. I felt like a normal 24-year-old."

After about three months of a fast nocturnal life, Kerry had burned through her rage and despair and emerged stronger. Reunited with Leighanna and her parents, she picked up the pieces.

Fast-forward and there she was in her kitchen, the most recent time I saw them, in March this year, with her arms around Leighanna, her peaches-and-cream daughter, who she says has been her reason to go on.

Kerry Needham, right, and her mother Christine speak to the media in October 2012. Photograph: Milos Bicanski/Getty Images

It was the morning after a late night. They had been at a fundraising party for the search for Ben in the local working men's club near Kerry's home in Ecclesfield. There's no question of giving up the search and they always need funds. We sat in her living room where your eye is drawn to a picture of Ben, frozen in time at 21 months old, the gravitational pull at the centre of their lives. Despite her lack of sleep, she looked beautiful – she has a wise, sad stillness that comes to people who have endured and survived.

She was wearing very little makeup, which isn't her idea of looking good. She goes for hair extensions, Botox and fake tan, the full disco diva look. But in her it's not vanity, it's making up for lost time. "I never used to take care of how I look," she said. "I've just been 'missing Ben Needham's mum' – it didn't matter about makeup or clothes as long as you're presentable. But I reached 40 and realised I was getting older. I needed to reinvent myself. I have to be me. I haven't had a life. I've never been 20 and I don't remember 21, 22, 23…"

Chris and Leighanna joined us. They, too, had had a late night at the benefit. Kerry and Chris were talking about the evening, amazed by the kindness of some Facebook friends who had organised it. The conversation meandered into chatter about who would play them in a film of their turbulent lives. Leighanna was easy to cast – Scarlett Johansson. But it was the idea of Jodie Foster as Kerry, having difficulty with a Yorkshire accent, and Ricky Tomlinson playing Eddie that made them laugh. That laughter, a tribute to their survival, is hard won. Kerry and Stephen were on a bus in Sheffield three years after Ben vanished. They laughed at some joke. A stranger approached them saying she was shocked that Kerry could laugh when her son was missing. She hopes her book will set the record straight: "It will give people insight into us as family, what we've been through, how we've been let down, how much of these 22 years we've had to do everything ourselves with no multi-million-pound campaign, no major police investigation. I'd like all those officials and politicians who could have helped us in the past to read it and go, 'Oh my God, what have we done?' And more than anything it will show Ben that we have never given up."

I have thousands of words of interviews with Eddie, Chris and Kerry, and a lot with Danny and Stephen, material gathered over the years. I have known them through breakdowns, splits, disasters and upheavals. Some might think I have been too close for objectivity, too partisan. But when I have heard accusations against them, I have checked them out. I was told by a woman on Kos that they sold Ben; they neglected him; Kerry was an unfit mother; they lived like hippies and Gypsies; they pretended to be poor but had a lot of money and two houses. Had there been any evidence for these rumours and I had ignored it, my affection for them would have been seen as compromising my integrity. The family were at first prime suspects and interrogated for hours by the police in Kos. In 1996, the police showed me the Needham file. There was no evidence lending credibility to the gossip, although there was a statement made against Kerry by someone she thought of as a friend, saying she had been with him the night Ben vanished. I asked her about it. She was with her family searching all night – she had alibis proving what he said was a lie. Why he did that remains a mystery. Three years ago I was writing about the family for Granta magazine. A British journalist told me he believed Stephen had accidentally killed Ben on his motorbike that day, and Chris had hidden his body. I challenged Chris. She asked me with bitter humour if I could see her doing that and lying for 19 years to her daughter, her husband, the world, herself. I couldn't and there wasn't any evidence against her.

These days, Kerry is as ready to talk as she was once silent and is strikingly self-aware. "I'm strong, I'm independent, I don't need anybody," she said to me that day in Sheffield in March, reflecting on her single status.

A digitally created image showing how Ben Needham was expected to look in 2007, aged 18. Photograph: National Missing Person's Bureau/PA Archive/PA Photos

Kerry's relationship with Simon ended when he went to jail shortly after Leighanna was born. She has had other relationships and more than her share of heartbreak – one man she loved was murdered just when they were about to get together. In 2006 she married Craig Grist, a builder, but they split up two years ago. "Craig has a heart of gold, he made me happy and we had good times, but he didn't get me. He didn't know how much I'd suffered. That's not his fault. I probably punished him in ways he didn't deserve. When it came to being serious and doing the things I had to do, he couldn't handle it. I don't think anybody could, that's probably why I'm on my own," she says, laughing. "Who could live with Kerry Needham and deal with the shit she goes through? You're having your tea and suddenly I have to go to London or Greece or the press turns up."

Kerry plans eventually to live in Turkey to get away from being stared at as Ben Needham's mother, or approached by strangers who ask her how she feels and whether there are any developments. Because of their good intentions, she feels bound to explain over and over again. "I am only doing what any mum would do," she says.

But in the supercharged world of instant global media and social networking she has a touch of celebrity status through hundreds of articles, YouTube clips of TV appearances and has been in Hello magazine and OK!.

The internet offers an infinity of possibilities. There are two Ben websites, a Ben Facebook page, Kerry's Facebook page, YouTube clips and many pictures of Ben on the web. Two young men have contacted Kerry on Facebook wondering whether they were Ben. They weren't.

There is a public investment in people whose fame is predicated on their suffering, and there are those who expect their suffering celebrities to behave like the Pietà – forever grieving, crushed, relinquishing identity and desire. On her Facebook page (4,109 friends), there are pictures of Kerry at parties and on holiday, wearing slinky dresses and bikinis. Someone sent her a Facebook message: "Look for Ben not going out." (sic) "How dare these people?" she fumed. "If I don't laugh and enjoy myself I'd die. Do they want me to die? It's the same when people say if it was me, I wouldn't be here. Should I feel guilty for being alive? I want to find my son and I will fight until the day I die to find him."

Nowadays she is in control of the search, liaising with Sheffield police who now have a team supporting her and assisting the Greek police on Ben's case. In February she and her mother invited me to accompany them to Thessaloniki and track down people the Greek authorities had failed to find after reports of sightings came in to a TV show she had appeared on last October. There were two people Kerry was determined to find: someone had contacted the TV channel giving the name of a Gypsy who she said was Ben. Another, a man from Veria, told Kerry via Facebook that he had seen someone in his local supermarket resembling the enhanced image of Ben. He named the man, also a Gypsy, who was married with two children.

One of her Greek Facebook friends, a young unemployed teacher, took us to the Thessaloniki police headquarters one evening and talked her way into the chief's office. He had the seen-it-all expression of the seasoned cop. Kerry was tense as she waited. Half an hour later he produced photographs of the two men. Kerry scrutinised them, shrugged slightly, showed them to Chris and me, and looked at them again. Neither looked how Kerry imagines Ben to look and bore no resemblance to the digitally enhanced photo fits of Ben as he might have aged, although one – a handsome young man with large, strangely bottle-green eyes – faintly resembled Simon.

Sometimes Kerry wonders if Ben has grown up in a Gypsy community and is reluctant to emerge or is under pressure not to, as the story of his kidnap would then come out. The police chief told her that's impossible. She's not convinced and suspects the police don't look in the Gypsy communities, which they say are no-go areas.

She was still holding out the faint chance that the handsome man with the bottle-green eyes might be Ben. Then she met him, accompanied by his father – the young man wasn't Ben. Kerry takes the disappointments with a sigh now, but it costs her emotionally every time. The blow was softened by the friendliness of the two men – the father had tears in his eyes. The other sighting remains unresolved but looks unlike the Needhams or the enhanced image.

People go on emerging, saying they have seen young men they think are Ben, often telling bizarre, disturbing stories. Kerry feels sure Ben is alive and that one day she will find him. "I may never have a proper relationship with Ben, but once I've found him I'll know he's alive and that will be enough," she says in her mellow, sad, wise way.