Wendy Parry cuts off the question mid-sentence. “Do you want to know who …?” Her answer is firm, her soft voice raised as she pulls her pink cardigan tighter across her chest. “No. We know what you are going to ask and the answer is no.” How many parents would not want to know who killed their child? Colin and Wendy Parry insist they have no wish to find out who was responsible for the murder of their son, Tim, who was killed in an IRA bomb attack in Warrington, Cheshire, in 1993. He was 12 years old. This Saturday would have been his 38th birthday.

Earlier this week, the couple received a visit from Judith Thompson, the Northern Irish commissioner for victims and survivors of the Troubles. She asked the Parrys how she could help. “I’d like to know why Warrington was chosen and I said that to her, but I don’t want to know who did it,” says Colin. “I don’t want to know what his name is, what he looks like, whether he’s got a couple of kids of his own. The whole idea that everyone wants to know who killed whom isn’t true. There are an awful lot of scabs going to be lifted as a result and I do wonder what the point is.”

No one has ever been prosecuted for the Warrington attack, in which two bombs planted in litter bins on a busy shopping street detonated within minutes of each other the Saturday before Mothering Sunday. Tim had gone into town to buy some Everton shorts and a Mother’s Day card. He died five days later, when the doctors asked the Parrys for permission to turn off his life support. A few weeks later, they went back to JJB Sports to buy the full Everton kit Tim was then buried in. His funeral was covered live on national TV, with well-wishers lining the streets for four miles from their house to the cemetery.

Flowers laid in tribute to the victims after the attack in 1993. Photograph: Richard Baker/Corbis via Getty Images

Until then, the IRA’s attacks on British soil had for the most part targeted wealthy parts of London: Harrods, Canary Wharf, Bishopsgate in the City. It is still not clear why they chose Warrington. It was home not to politicians and tycoons but to ordinary people such as the Parrys: Colin, an HR director, Wendy, a school meals supervisor and their three children. Their “Timbo” was not the only child killed: a three-year-old boy called Johnathan Ball also died. Fifty-four people were injured.

There is nothing to be gained from putting someone on trial, says Wendy. It won’t bring Tim back. “We just want to carry on what we are doing, and doing positive things.” Positive things for the Parrys are primarily related to Warrington’s Peace Centre, a residential facility for young people that also serves as the base of the foundation they created 20 years ago in the name of the two children killed in the attack. It has a cafe, a sports centre and dormitories, and runs training sessions for young people to explore the foundations of extremism, encouraging them to think critically about community tensions. Warrington council originally wanted to put up a statue to the two victims, but Wendy didn’t like the idea: “I thought it was the sort of thing that, after a couple of months, people wouldn’t look at or know why it was there.”

Instead, a trip to a “peace farm” in Northern Ireland, where young Catholics and Protestants lived and worked in harmony, provided the inspiration for a “living memorial”. They visited just a few months after the attack, for a remarkable edition of Panorama that they made with the BBC journalist Nick Robinson, in which they travelled to meet Protestants and Catholics on both sides of the Troubles.

They made friends in high places. Mo Mowlam, the then Northern Ireland secretary, helped raise the money for the peace centre. With the help of Tony Blair and Bill Clinton, Mowlam tapped up Charles “Chuck” Feeney, an Irish-American described by the New York Times as “the James Bond of philanthropy” for the secretive way in which he gave away his billions. The Tim Parry Johnathan Ball Peace Foundation is a nonpolitical, nonprofit organisation that works nationally and internationally for peace and nonviolent conflict resolution.

Initially, it ran exchange trips for young people from Dublin, Belfast and Warrington to meet and become friends, but recently has become something much bigger. Last year, it received £400,000 from the Home Office as part of the controversial counter-extremism programme Prevent. (Prevent has been criticised for focusing on certain groups, but Colin says: “It’s better than nothing. I think if white supremacists think it’s skewed and Islamist extremists think it’s skewed then it’s probably about right.”)

It also won a £500,000 grant from the Big Lottery Fund to teach conflict resolution skills in three of the north of England’s most divided communities: Harehills in Leeds, and Rochdale and Oldham in Greater Manchester.

The foundation works closely with the victims of terrorism, too. In the past 18 months, it received 900 new clients following the attacks in Manchester, London and Paris. When I visited this week, a group of teenagers were making care packages to send to the survivors of the Manchester Arena bomb. “This …” says Colin, gesturing to the surprisingly large, wood-clad building that they share with the children’s charity the NSPCC. “This is what keeps Wendy and me sane.”

A lot of people have said: ‘Why would you want to bring all that pain back?’ And I’ve said: ‘It never goes away’

The foundation is why the Parrys agreed for their story to be turned into a BBC Two prime-time drama, Mother’s Day, which will be broadcast on Monday. It is written by Nick Leather, a TV scriptwriter and Warrington local who was 15 when the attack happened. Contrary to what some critics in Warrington’s local paper have said, the Parrys were not paid for their involvement. They just hoped the drama would get the word out about the peace centre and so they were sad that the script ends 40 days after the bomb attack. “Of course we were disappointed,” says Wendy. “But we still think they have done an amazing job.” She was pleasedto hear that the BBC had agreed to add an endnote to the 90-minute film, making reference to the foundation and its work.

The story is told from the perspectives of Wendy, played by Motherland’s Anna Maxwell Martin, and Susan McHugh, the Irish nursery worker who was galvanised by the Warrington bomb to start a peace movement. McHugh’s campaign led to a 20,000-strong protest in O’Connell Street, the site of the 1916 Easter Rising in Dublin. She is played by Vicky McClure (Line of Duty, This is England), while Colin is portrayed by McClure’s Line of Duty co-star Daniel Mays.

At the time, Wendy’s voice was seldom heard. It was Colin, a garrulous scouser, who spoke to journalists each day at the hospital. He does most of the talking today, too, teasing his wife that the scriptwriter had to invent her one big speech of the film, on an Irish talkshow, because “she was as quiet as a loaf of bread back then”. When she first watched Mother’s Day, she worried that Martin had made her “too hard”. No, no, said the director, Fergus O’Brien, who has a background in documentaries. “He said he was telling the story of a woman who was going through so much and was trying to hold it all together.” Colin thought Martin had got his wife exactly right. So did Abbi, the Parrys’ 36-year-old daughter (they also have a son, Dominic, 39).

“Wendy was the one who kept the family together, fed the kids, got them to school, told them to go to bed, told me to go to bed, fed me, got me off to work. She was the one who held it together. She was grieving privately, of course, but I was the one who was blubbing a lot and who was incapable of any decision-making,” says Colin. “Most mothers, when a disaster or crisis happens, do that. Women take control. They are the anchors.”

Anna Maxwell Martin and Daniel Mays as Wendy and Colin Parry in Mother’s Day. Photograph: BBC/Steffan Hill

Abbi, who has two children of her own, did not want her parents to consent to the drama. “She was saying: ‘Why are you doing this? Why are you dredging up the past? There’s nothing to be gained from this.’ That was once it was clear the peace centre wasn’t going to be [part of the film],” says Colin. Wendy winces. “A lot of people have said: ‘Why would you want to bring all that pain back?’ And I’ve said: ‘It never goes away.’ For Colin and me, it’s always there. It’s not as raw as it was 25 years ago, but losing a child never, ever goes away.”

They think about Tim every day. “You are never cured,” says Colin. They are forever thinking what would have been, says Wendy: “It’s upsetting for us now to see his best friends getting married and having their own children. Even our children having their children, you think: ‘That’s the sort of thing Tim’s missed.’ He has missed going on the great family holidays we’ve had, getting married, having children, having a career.” One thing is for sure, says Colin. If Tim had lived, “we wouldn’t have had four grandchildren. We’d have had 10.” Tim, he laughs, might have only been 12, but he was already very interested in girls.

Most of the film is accurate, the couple say, although Colin says he never wore a vest in bed (“These northern stereotypes!”) and a scene in which Wendy goes back to her job as a school meals supervisor at Tim’s school and drops a tray of pies was invented. They say it slightly overplays their relationship with McHugh, whom they have seen perhaps half a dozen times in the past 25 years – they are really just on each other’s Christmas card lists, says Colin.

Watching the film, you may wonder why we hear nothing from the parents of Johnathan Ball. The reason is that they are dead. They never recovered from the loss of their son, the Parrys believe. Johnathan’s mother, Marie, returned to her first husband after the bombing. His dad, Wilf, “had a very sad end to his life”, says Colin. “He was mostly seen sitting in the centre of Warrington on his own.” Wilf was a trustee of the foundation for a few years, but only came to one meeting. “It wasn’t for him,” says Colin. The last time they saw Marie was when they were visited by Martin McGuinness, the Irish republican Sinn Féin politician who became the deputy first minister of Northern Ireland. “She fell asleep while we were talking to him,” recalls Colin.

The Parrys are certain that their son’s death hastened the peace process, which saw a ceasefire declared on 1 September 1994, what would have been Tim’s 14th birthday. “The shift of public opinion in Ireland was huge: 20,000 people stood on O’Connell Street. Ten times that many were outraged. Thousands wrote to us. I have no doubt that fed into Irish government policy and there was a new impetus to start things moving,” says Colin.

The then prime minister, John Major,a patron of the foundation, told them his first instinct was to cancel any clandestine talks. “Then he thought: no, actually, this means the talks have got to carry on and efforts have got to be redoubled,” says Colin. “The pebble in the water that was Tim and Johnathan caused a tsunami that in the end led to a whole new approach.”

Mother’s Day is on Monday 3 September at 9pm on BBC Two.

• This article was amended on 3 and 7 September 2018. In the 1993 Panorama programme cited, the Parrys met Protestants and Catholics on both sides of the Troubles, not the parents of IRA terrorists as an earlier version said. Also, this programme was produced by John Bridcut, with Nick Robinson as deputy editor. This article was further amended because the name of a former commissioner for victims and survivors for Northern Ireland, Bertha McDougall, was inserted during the editing process, rather than the current commissioner, Judith Thompson.