One spring day in 2001 a tall man walked into Sheffield’s Park Hill flats and along a street in the sky. He strode past the brutalist flanks, out on to the footbridge. He thought: this’ll do.

Jason didn’t look down; he gets vertigo and he was 13 storeys up. He leaned over in his yellow Puffa jacket and sprayed her name. “Clare” came out haphazardly and “Middleton” hit the ledge. He planned to take her to the Roxy on the facing hill, to show her. So now he began again, bigger, clearer: “I LOVE YOU WILL U MARRY ME”. It was his two-fingers-up at the social services office opposite. He scarpered. Seeing it, Grenville, one of the estate’s caretakers, said to the on-site office: “How are we going to get that off?”

They didn’t. The graffiti stayed, high above the city, while the city argued about what to do with the flats. Park Hill, the concrete estate behind the railway station, had become notorious. The city projected abandonment on to Park Hill, so the graffiti started to look like love yelling at the top of its voice in an estate thought to be desolate.

Soon it was also looking like PR. In 2006 Sheffield City Council appointed regeneration specialists Urban Splash to redevelop Park Hill. From the start, the company used the graffiti as a slogan to woo Sheffielders back to the epic, undemolishable council blocks of 995 flats, Europe’s largest two-star listed building. “I love you will u marry me,” sang the flats. Clare was edited out.

“She had difficulty expressing love’: Clare Middleton as a young woman.

Jason didn’t know his proposal had become something else. He didn’t know that Professor Jeremy Till, then at the University of Sheffield, jauntily displayed a replica of the bridge at the 2006 Venice Biennale of Architecture. Or that Urban Splash had replicated the proposal on T-shirts for their launch party and one of the Arctic Monkeys wore one on stage in America.

One day in 2010, I looked up and wondered who the lovers were and whether Clare said yes. I asked film-maker Penny Woolcock to work with me on a documentary for Radio 4, a quest to find the real lovers behind the graffiti. We knew nothing. I rhapsodised to the local paper, The Star, hoping that we would uncover a romance. I trawled local news archives and we chased up rumours. There was one story that Clare Middleton said no to the proposal, so her spurned lover had jumped off the bridge. The Park Hill caretaker, Grenville Squires, said not: 26 other people did jump off the bridge, one of whom Grenville heard yell “help” just before he jumped. But not the graffiti man.

A fork-lift driver had heard it was a love triangle and the person who wrote it had his flat burned out by the other bloke. Grenville said no. He did remember “one or two house burnings on Park Hill but most of them’s been by accident”. He told Penny that brides rode down to their weddings in the estate’s service lifts so that their big frocks didn’t squash. But none of the brides who descended that way was Clare.

An artist called Emilie Taylor used to watch drug users snake under the graffiti when she worked on the Needle Exchange van that parked under the bridge. She asked around, because it seemed an intoxicated act, that proposal. But – nothing.

Without answers, I recorded texture. On the bridge I recorded wind, trams, sirens: “There’s always a siren,” Jason said when we finally met him. I recorded children in the playground of the nursery on Park Hill. It turned out to be the nursery where Clare Middleton played as a child.

That Easter, 2011, I got an email from Tim Middleton: “Regarding your article in the Sheffield Star… Clare Middleton was my step-daughter.”

‘There has been incredible change at Park Hill’: the grade II-listed flats as they are today. Photograph: courtesy of Urban Splash

Tim has Love and Hate tattooed on to his knuckles. He was a Hell’s Angel when he married Janet. She had a toddler called Clare. They moved on to Park Hill. Tim, now single, keeps a very clean house on a red-brick estate and his front garden is full of blooming white pot plants. His ex-wife Janet joined us there, and they talked about Clare, dark-skinned and skinny, doing cross-stitch, reading Catherine Cookson, being happy, going clubbing.

Tim thought the graffiti was a nonsense: “Graffiti on a listed building!” They weren’t in touch with the man who’d written the proposal. Janet said: “She was going to marry him but circumstances…” She asked me to turn off the recorder. There were so many things we couldn’t say.

We met Jackie, Clare’s half-sister, in what she calls her “bubble” on another hill in the city. She says Clare loved the proposal, “Best thing anyone’s ever done for her.” Clare was living alone with her two boys, a toddler and a baby, when she met Jason. Penny asked, “Did you love Clare?” Jackie said yes. “I did love her. I didn’t like a lot of what she did.”

When she was young Clare craved love, Jackie said. She devoured Mills & Boon. “She used sex and love as a crutch to prove that her life meant something.” She would go dancing in thigh-length boots and hot pants. She attracted “bad boys”. Jackie described Clare’s life as “a spiral” – spiralling, like she danced.

Teenage Jackie had wanted her older sister “to stop”. “Stop what?” asked Penny, “Stop flitting, settle down, be a mum.” It wasn’t that Clare abandoned her children, says Jackie; it’s that she took them with her through her various romances, “flitting, flat to flat”. But on the day Jason led her to the Roxy opposite Park Hill, it was just the two of them. Clare opened her eyes and said “Yeah” to him, “Because Clare wanted nothing but love.”

Jackie put a note saying “someone’s looking for you” through Jason’s old letterbox. It was 10 years since the Middletons had seen him. Back at Park Hill, we climbed the flights of stairs and on to the bridge. Gangly Jason, 6ft 6in (which explained how he could lean over and paint on such a precarious spot) looked out at the city. He said when he met Clare, it was like they recognised one another. They liked walking into town and out again, with the Alsatian.

She had a panther tattooed on her back with the claws ripping through her skin, and her children’s names on her hand. He attributed her lack of teeth to amphetamines; Jackie says it was due to “sheer neglect”, that Clare didn’t take speed until later, until after she’d had problems with social services.

Jason wrote his wild love plea on the bridge because, “Even though she was a loving person she was not one for accepting love.” Just asking her face to face wouldn’t have been enough to capture Clare. And she said yes. “But things went downhill from there.” Social services had advised Clare to pay attention to her children, not Jason. “Were they thinking you were a threat to the children?” asked Penny. Jason grew up in care, “Then I was lobbed into my own flat when I was 15… and left to fend. I have had a lot of dealings with social care and I don’t play by their rules, and they don’t like it. It’s caused me hassle with my own children and in relationships I’ve been in after, too.”

Pillow talk: Jason’s words used on cushions in one of the flats. Photograph: courtesy of Urban Splash

Jason was abused in children’s homes, both physically and sexually. He trembled, talking to us, as if he was cold. He said that social services warned Clare off him because they believed it probable that someone abused in childhood would become an abuser. His own children were taken into care. When Jason scrawled the proposal in social services’ eyeline, it was the beginning of the end. “If you’re told you’re going to have your kids taken off you, you’re going to choose your kids, and that’s what she did,” says Jason. He was distraught. “He became her stalker” for a while then, Janet told us.

“How could you know how to be with children in an ordinary, loving way?” asked Penny, given how he’d been treated. How would he behave with children? “Treat them the exact opposite of how I was treated, basically,” he said.

Jackie was clubbing with Clare one night when a friend came and said, “Your Clare’s doubled over.” Jackie found her in pain and bleeding. It was cancer.

The man she had gone on to marry couldn’t cope. Clare went home to Janet’s to die. She said goodbye to her children. Tim said that whenever he came to visit her she never complained about anything. “Whatever it was she accepted it – even when she was dying.”

When we heard that Clare died aged only 30, Penny and I felt differently about the graffiti – we were intruding on grief. “I always hoped it would be washed off. It’s not a romantic gesture to me, it’s a reminder of Clare, every day,” Jackie told us. “I’d like to go about life pretending that nothing’s happened. And you can’t do that when her name’s scrawled in such a public place.”

Jason doesn’t regret anything. “I know Clare had difficulty expressing love but she did it enough for it to be a godsend to me. Because, like I say, I’ve not received it.”

Just as we finished recording in 2011, Urban Splash, inspired by Tracey Emin and facing a conservation dilemma – what to do with fading graffiti on a listed building – overwrote Jason’s handwriting in neon. The “I love you will u marry me” lit up as urban art, 10 years after Jason painted it.

Urban Splash’s manifesto for Park Hill had justified editing out Clare’s name from the moment they started to redevelop the estate, back in 2006, saying they believed it was added by a freeloader. “We think that Clare Middleton’s name was added after,” they said. “We found out about her. She’s just got divorced.” I felt puzzled that they didn’t seem to know that real people were at the heart of the graffiti. It’s because it is real that it works.

Buyers of new flats were given a bottle of I Love You beer in their welcome pack

They replicate Jason’s proposal on sales posters for the flats and on the sales office’s glass doors. It has been embroidered on the show flat cushions. Businesses borrow it, too. People drank I Love You Will U Marry Me strawberry beer made by Thornbridge, a local brewery, at this year’s fund-raiser for the onsite S1 art space, and buyers of new flats were given a bottle of the beer in their welcome pack.

“Jason’s reckless, crazy gesture has been appropriated for profit,” says Penny. “I Love You has been drained of meaning, transformed into a hipster brand advertising gentrification.”

People send Jackie hashtags to the stream of graphics that have been inspired by the graffiti. She deletes them. She sits behind people on the bus “oohing and aahing” about the proposal and says nothing. After our documentary was broadcast, Urban Splash went quiet. Now, five years on, Mark Latham, regeneration director, is thoughtful about their use of the graffiti: “It wasn’t deliberately, knowingly, an intervention in a family grief. But it was on a building and had been up for years,” he says. “Once art’s original purpose is done it can take on other meanings.”

In the five years since we recorded, there has been incredible change at Park Hill. The massive lower flanks of flats have been Urban Splashed; revived to the architects’ original brutalist genius, with added glass and bright aluminium panels. In May, Warp Films joined select, groovy companies who have bought cool concrete offices in the mix. Warp filmed desperately forlorn scenes for This Is England just metres away, on the other side of the bridge, where there are about 600 ex-council flats, derelict. The I Love You bridge links those two worlds: the old and the new, the dejected and the done up.

Whereas 63 of the 260 flats in Phase 1 of Park Hill are social housing, it looks like there will be no government subsidy for any “affordable” rented housing when the flanks on the far side of the bridge are redeveloped. If this is true, the price of saving Park Hill will be that only people who can pay for it can live there. The alternative for the estate, says Latham, is decay.

The whole city sees the bridge when we look up. It lives with us in myriad ways. Travellers see it, glowing blue at night behind the railway station. Young people see a love story. To Jackie, “It’s bittersweet. Clare was proud of the bridge to the day she passed away, but to me it’s a ghost that will live on the skyline of Sheffield for ever.”

Jason alone is delighted about his graffiti’s afterlives, especially the Arctic Monkey T-shirt. Delighted that people borrow his proposal to help make their own, standing under the bridge to pop the question or sticking wedding rings on to bottles of I Love You beer. He is also absolutely broke. He says he wrote an email in 2014 to Urban Splash saying something like, “You’re making all this money out of my graffiti and I’m homeless. Can you give me a flat?” He says there was no reply.

I drove Jason to Park Hill to have his photo taken for this article. In his own words, he’s “not in a good place”. He’s tracked his father down, only to be rejected. One of his sons has been sent to prison for life, for murder. He heard the sentence announced on the radio while at work. Soon he left that job, near breakdown. He struggles to get work because of an unspent conviction (for harassment). He moved house for his last job, and he is struggling to meet the rent: on the phone in the car he pleaded to a Universal Credit helpline, in rising despair and tears.

At Park Hill the Urban Splash PR took his number, offering to see if there was any I Love You beer left in their office. She texted after to say “really sorry – the sales office has run out.” He replied, “They could give me a flat if they like as I’ll be homeless in four weeks.” She wished him good luck.