As noted in our review from last month’s Glasgow Film Festival, Oranges and Sunshine is a “fantastic, powerful movie”, based on the true-life story of social worker Margaret Humphreys.

In the 1980s she uncovered the enforced emigration of a horrifically vast number of children over to Australia and other countries in the British Empire, the circumstances of what happened to them both before and afterwards shocking to comprehend.

However, the story is handled subtly and sensitively by director Jim Loach and Scottish screenwriter Rona Munro, who were also in Glasgow in February to attend the screening of the movie at the film festival, and who talked to us about its development.

Loach explained: “I went up to Nottingham and met Margaret, and spent a lot of time with her, and just found her story utterly compelling and completely inspirational. Every time I saw her she’d tell me another story which beggared belief.

“You couldn’t believe it had happened, and you couldn’t believe so little had been said about it. From that moment we knew it was a movie that we wanted to make.”

Munro added: “I mean, I hadn’t heard about it at all. A lot of people were in that position, and a lot of people have the same reaction, which is that it’s such an incredible story. For it to happen within your own lifetime and not even to know about it is quite extraordinary.”

It was the first feature film for Jim Loach, son of celebrated director Ken, who previously had worked on TV shows including Coronation Street and Shameless. Aberdeen Rona Munro has meanwhile written for a variety of media, and among the likes of Doctor Who and Aimée & Jaguar was indeed the author of the screenplay for Ken Loach's Ladybird Ladybird.

The casting of Emily Watson in the main role of Margaret wasn’t too difficult, and as Loach joked: “We had a list and she was at the top, and we were very lucky that she agreed to do it.”

He continued: “We really wanted Emily because she has that combination of compassion but without sentimentality, but also she’s a very strong, very intelligent woman, and she’s a lot of fun. She brings all of that to the part of Margaret.”

As well as Watson, there’s a strong supporting cast that includes Hugo Weaving and David Wenham. Loach explained: “It was a tribute to the script that they were really keen to do it. Both the characters were drawn from real people that we met.

“David Wenham plays a character called Len, who is the opposite of a victim. If you offered him sympathy then he’d tell you to get lost. He’s not interested in your kindness, he’s a very brash man but underneath a very complex person, and those layers get stripped away during the story.

“Hugo plays a character called Jack, who is the absolute opposite of Len. He’s clearly a very damaged man, and very, very fragile.”

So what did Margaret think of the movie? Munro said: “She seemed to like it; I mean, she did like it. We were very nervous about that screening, as you can imagine, and I think when she came out we were just staring fixedly at her face for a long time just trying to register, had she liked it, had she not liked it.

“When she relaxed and told us what she thought the word she actually used was ‘faithful’, which I think was the biggest present she could have given us, because that’s what we were really trying to do.”

Oranges and Sunshines comes out in UK cinemas on Friday April 1.