For decades Britain forcibly shipped children in care to Commonwealth countries. They were sometimes even told their parents were dead – many were used as slave labour, many were abused. The scandal was discovered by social worker Margaret Humphreys

In 1986, Margaret Humphreys was a social worker in Nottingham, specialising in child protection. One day an Australian woman contacted her, to say she was trying to find her mother. The woman said she had been taken from a children's home in Nottingham and sent to Australia by boat, aged four, during the 1950s. Could Humphreys help? It was to lead to a quest that would take Humphreys across the world and uncover a scandalous policy used to forcibly ship thousands of British children to Australia and other Commonwealth countries after the second world war.

By chance, another woman in a post-adoption support group run by Humphreys had recently mentioned a brother in Australia. This woman hadn't seen him since he was taken from the children's home where they had both been living. The two incidents sparked Humphreys' interest but when she asked an official at the Australian high commission in London if they had any information about children who had gone to Australia during the 1940s and 50s, she was referred to a senior member of staff. "He said: 'These records were sent to Canberra several years ago,'" Humphreys remembers. "I was absolutely stunned. These records? That meant there had to be far more children involved."

She placed advertisements in Australian newspapers, asking people with similar stories to come forward. A dozen or so did. Many of the children had been given new names and told that their parents were dead. "We soon discovered that the parents hadn't died. They were very much alive. And that was quite a revelation – and quite a moment."

Twenty-five years on, Humphreys' extraordinary story – and the story of the child migrants she has helped over the years –has been made into a film, Oranges & Sunshine, in which the role of the social worker is played by Emily Watson. However, movie-land is far removed from Humphreys' life: she is still in Nottingham and her office, in which we chat, is above a sandwich shop near Trent Bridge cricket ground.

Many of the children – mostly aged between seven and 10, some as young as four – became child labourers on farms. It wasn't the lives of sunshine and plenty they were promised by the officials who packed them off on a boat as if they were heading for an adventure holiday. Cut off from their families, they were intensely vulnerable and some were subjected to appalling physical and sexual abuse; one destination, Bindoon, near Perth, run by the Christian Brothers, became particularly notorious. As one boy, who spent his childhood there and helped to build the dormitory he slept in, said: "My blood is in those stones."

It's difficult to understand why such a misguided policy – which ran from the 1930s to the late 1960s – was ever allowed to happen. But the fact is that the British government, along with the Australian government and various local authorities, church groups and Barnardo's, took part in an operation aimed, largely, at getting rid of a costly social problem – the migrant scheme almost invariably involved children from deprived backgrounds who were in some form of social or charitable care – and solving Australia's problem of a dwindling workforce. Many of the children were told their parents were dead; their parents were given scant information about where the children were going and many didn't even know they had left the country.

Did Humphreys – who founded the Child Migrants Trust to help people who suffered under the policy – have any idea of what she was getting into when she started to uncover the scandal? "No, not at all," she says. "We never knew it would be this big or that we would still be doing it more than 20 years later. But we got to a point where it was so wrong and so awful that you knew you had to take it to the government, who we thought would do something straight away. And then when they did nothing, you had to carry on."

In the film, Humphreys' son, Ben (she also has a daughter, Rachel – both are in their 30s), is asked at a fundraising Christmas party what he has donated to the raffle. "I gave you my mother," he tells them, hinting at the fact that – then aged 12 – he has hardly seen her all year because of the hours she has worked for them. Did she become obsessed with her job – and did her own family suffer? "It was never an obsession," Humphreys maintains. "But I was very focused on what I was trying to do. But each time I thought that would be the end of it – I'm going to find this person's birth certificate, or I can reunite this woman with her mother – and then go back to my normal life. It never turned out like that. Every time, something else happened – more people coming forward, more information about the policy."

Do her children feel they suffered because of how hard she worked? "Would we as a family look back and say we wish this would have been different … ? They were in a family of social workers [her husband, Mervyn, is also a social worker] and knew that family was at the heart of our lives."

Humphreys, 66, didn't visit the set during filming or offer her services as a consultant. "I'm a social worker, not a celebrity," she says. "I took the view that we're both professionals and I have my skill and Emily [Watson] has hers. I trusted her and Jim [Loach, the director] to do right by us, and they did."

Humphreys took some persuading to agree to the film. Loach had read her book, Empty Cradles, and contacted her several times about making a film but she demurred as she didn't want the story of the migrants to be about her rather than the people who had suffered under the policy. Then, she reconsidered. "Jim came at the right time – the political developments were slow, the funding was abysmal and I thought this [film] would get the message through. Every week you were seeing people dying [the parents of migrants, many by now elderly]. And you don't do a thing like this for yourself, it's for those people there."

She gestures to a board that fills almost a whole wall, on which are pinned hundreds of photographs of child migrants, many taken at events that reunited them with their families.

Coincidentally, soon after filming started in autumn 2009, the then Australian premier, Kevin Rudd, made an apology in the Canberra parliament for the child migrant policy. A few months later, the British prime minister, Gordon Brown, also apologised in the House of Commons, and money is now forthcoming in both countries for the trust's work. Humphreys was also made a CBE.

She expects the film to encourage more migrants to come forward – she guesses that the trust has had contact with about three-quarters of the estimated 3,300 children shipped to Australia since the second world war – but it is, she says, a problem that will take another few generations to sort out.

Are the family reunions happy occasions? "People may think they are wonderful but it's not that way at all because you know, at that moment, all that's been lost. It's quite profound, realising what needn't have been."

Of the thousands of people the trust has helped, the person who most stands out, says Humphreys, was the son of a child migrant who came to see her after his father's death. "He had nothing but anger and sadness and bitterness, and we helped him understand those feelings and why his father had been so harsh. At the end of that process, he took his dad's ashes back to where he was born in Britain because that's where he wanted him to rest.

"That will always stay with me because it was about understanding and forgiveness – exactly what the trust is about," she says.

In the late 1980s, Humphreys' health deteriorated for a while and she was diagnosed with post-traumatic stress disorder. "It was a huge challenge, but I think I'd be less than human and I couldn't do my job as effectively if there wasn't an emotional cost, because you can't stand outside of everything – you wouldn't be human if you could." Taking into account all it took, was the personal cost worth paying? There's a pause before she says quietly: "I don't know … I don't know."

Seeing my surprise, she immediately rallies. "But I'm still standing – and there's a lot more work to be done."

Oranges & Sunshine opens this weekend

More information, childmigrantstrustcom