ALTHOUGH Margaret Humphreys took some convincing to allow director Jim Loach to make a film of her work, uncovering the scandal of the child migrants, for Oscar-nominated actress Emily Watson it was a given.

"It's an amazing story, just very, very powerful, compelling stuff," says Watson, who stars as Margaret in Oranges and Sunshine, which opens in cinemas today.

"It was two years ago when I met with Jim on a very snowy day in London. We sat and talked about it for hours. It was one of those things that just felt right from the off."

The film tells the true story of how Margaret first discovered that children in care in the UK were sent to Australia and other Commonwealth countries with a promise of a better life. Around 130,000 were deported between 1920 and 1970 and many endured abuse.

"I hadn't heard of them at all," says Watson of the child migrants.

"I think it's been a very, very little known fact. It's better known in Australia obviously because the migrants are there, but in England, no, I'd no idea. It's outrageous really. Now it's obviously coming much more into the public eye because of the apologies in both countries."

Last year both the Australian and British governments publicly apologised.

Oranges and Sunshine opens in 1986 when Margaret, then a social worker in Radford and Hyson Green, is approached by a woman.

"She says she was sent to Australia from England as a small child without parents or guardians," says Watson.

"Margaret's first reaction is disbelief. So she starts following the trail and uncovers this incredible story – the woman is just the tip of the iceberg. When she first starts looking into it back in the 80s her boss says to her, 'Do you want to take a year or two to do this?' – and in reality it's been her entire life ever since then. It takes an incredible toll on her physically and mentally and emotionally. She tells herself that she's keeping her distance, that she has boundaries, but in fact she is diagnosed with post-traumatic stress disorder, just because of the emotional impact of where she'd been and what she'd been hearing."

To prepare for her role, Watson, 43, who won an Oscar nomination for her role as Jacqueline du Pré in Hilary and Jackie, resisted meeting Humphreys.

"I thought long and hard about it and every day when we were filming I would say to myself, 'maybe I should have met her,'" says the actress, who has also appeared Cradle Will Rock, Angela's Ashes, Gosford Park, Red Dragon, Punch-Drunk Love and Equilibrium.

"I've played real people before and in a way you almost get too close. It's very difficult to be objective about them.

"Also, the way they are physically, the way they sound, becomes a very strong imprint and sometimes that's not helpful when you're trying to tell a story. You end up just trying to mimic them."

She did read Margaret's book, Empty Cradles, which has been republished to tie in with the film's release.

"For me the most important territory is the emotional stuff and that comes from having your own family and children," says Watson, who has a two-year-old son.

"It was a case of putting yourself into that imaginative area of what it would be like to be dealing with all this stuff.

"There is something utterly compelling about the thought of your own child being abandoned and deported and sent to an abusive children's home for ten years."

Oranges and Sunshine was shot in both Nottingham and Australia and she admits the climate change was a challenge.

"I think Jim had a very strong image before we started of how he wanted her to look – professional, sharp, self-assured. So I spent a lot of time wandering around in the blazing heat in Australia, very unsuitably dressed in a suit and stockings and heels and dying of heat," she laughs.

"But that's her look: she's quite buttoned up."

Which helped when the shoot came to Nottingham in November 2009.

"It was pretty cold. All the Australian crew were shivering and complaining," she laughs.

Her co-stars are Hugo Weaving and David Wenham, who play former child migrants Jack and Len.

"Jack just seemed to me to be a man crying out to be accepted and to have his story recognised," says Weaving, better known as Agent Smith in sci-fi blockbuster The Matrix.

"When we meet him he's still going through a great deal of issues and a lot of pain. Initially he's very mistrustful of Margaret."

He adds: "I wanted desperately to meet the man on whom Jack is partially based – someone who'd actually come out and had that experience. He was incredibly forthcoming. It was an invaluable experience talking to him and that was my primary research.

"The title Oranges and Sunshine is something that Jack says. He was asked as a child whether he wanted to go to Australia, where he could live in a white house, ride a horse to school and be able to pick oranges off the trees for his breakfast and where the sun shines every day.

"That was the sort of golden promise that these children were sold. Oranges and Sunshine? It's the great promise and the great lie, the great untruth that was told to these innocent children who were damaged for so many years.

"And it's the journey that Margaret takes to try to heal that and give them some sense of who they are."

Read our verdict on Oranges and Sunshine: Film reviews – page 11.