‘Daddy, my Daddy!” With these three words at the climax of the drama, Jenny Agutter 50 years ago created one of the best-loved roles in British cinema – as Roberta in Lionel Jeffries’ adaptation of E Nesbit’s 1906 novel The Railway Children. Now Agutter, who is about to start filming the tenth season of the BBC drama Call the Midwife, in which she plays Sister Julienne, has reprised her role in the story of the three children who move from London with their mother to a dilapidated cottage in Yorkshire, suddenly impoverished after their father disappears. This time she is taking part in a remarkable project by a charity set up to help rehabilitate prisoners into society.

Her involvement – which comes as the 1970 film is enjoying a new lease of life on the BBC iPlayer – reflects her own commitment to working with prisoners and children from disadvantaged backgrounds, which stretches back to her youth.

The actress has recently become an ambassador for Liberty Choir UK, a charity whose work begins inside prisons – through weekly rehearsals of a choir made up of prisoners and volunteers from community choirs – and which blossoms when the choir’s ex-offender graduates leave prison and are supported by the team and volunteers.

The charity has been grappling with its role in the coronavirus pandemic, with prisons not allowing groups to congregate and with singing forbidden. How can it replace that human bond, where for two hours every week people gather in prisons for conversations, and care for one another’s lives?

The charity – which I co-founded in 2014 with my wife, MJ Paranzino – has had to replace that physical engagement with the men and women in prison with virtual support. Two weeks after lockdown was imposed, Liberty Choir Radio was launched – a one-hour show of musical history, chat and songs, with breathing and vocal exercises (such as in the live sessions) and – so crucial, this – messages of hope and solidarity from the volunteers, supporters and the Liberty Choir graduates themselves who know so well what lockdown feels like, and can imagine this extreme version of it; some prisoners are only getting out for 30 minutes at a time, and have to choose between a shower or a phone call.

Liberty Choir Radio is also being launched with National Prison Radio next month to take the charity into every prison in England and Wales – more than 100, with an audience of 80,000 prisoners – so the charity’s delivery has actually expanded in this frustrating time, albeit nothing beats being together and singing together in a room.

Our new ambassador was asked if she would record a message for the prisoners and also read a passage from The Railway Children. The line everyone recalls is when Roberta (nicknamed “Bobbie”) first sees her much-missed absent father through the steam on the platform of the local railway station. He has been in prison for so many long, anxious months after falsely being accused of being a spy, and when his oldest and most resourceful child realises it is him, she runs into his arms weeping “Daddy, my Daddy!”. Well, how wonderful for those people in prison to hear such a touching reading by Agutter – some of them will know the story and for those who don’t, the words would still be meaningful to them.

Edith Nesbit, the author, was a political activist, co-founder of the Fabian Society in 1884, a lifelong socialist and unconventional – she was seven months pregnant when she married and lived in a ménage à trois. Her father died when she was three and she felt his absence keenly all her life.

What tugs at the heartstrings, hearing Agutter read the passage so movingly, with the same slight catch in her voice as her teenage self had 50 years ago, was the line about how the father feasts his eyes on the flowers in the garden, such a contrast to the ragged tufts of grass erupting from the cracks that was all he could see through his prison cell bars.

Agutter has been engaged in social issues and volunteering all her life. She is a patron of the St Giles Trust, which supports prisoners and their families as well as other vulnerable groups.

In her early twenties, having acted since she was 11, Agutter moved to west Hollywood. Not long afterwards , she started volunteering at a school, Sheenway, in Watts, south Los Angeles, which was and still is a poor African American neighbourhood. Only 10 years earlier there had been riots where 34 people died and thousands were injured.

It was a fellow actor who asked Agutter to take over from him at Sheenway, but what made her decide it was something she wanted to do? “I just hooked into it immediately as something that intrigued me, partly because it was a very different place to where I lived and also because I had been given so many opportunities – whether it was doing The Railway Children, or getting into theatre and ending up at the National Theatre, or going to America and doing Logan’s Run,” Agutter said. “And I’ve been very grateful, but I’m also very aware that many people do not have opportunities in their lives. So there’s a part of me that has a strong sense of ‘I can’t just live by taking stuff and not giving stuff back.’ ”

Agutter recently sent an email to Dolores Sheen, the 82-year-old executive director of Sheenway school (established by Dolores’s father, Dr Herbert Sheen) in a gesture of friendship which goes back 40 years. “It was after George Floyd’s death,” said Agutter, “and I said, ‘I hope you and the kids are doing OK, and I’m so sad and sorry that these things we thought would have passed by now have not passed.’ ”

Her “giving stuff back” was to introduce the schoolchildren to Shakespeare. Agutter recalls taking a group of 10 to see Richard III which was playing in an affluent part of Los Angeles. The head had given permission and a coach was organised and then “as the children are leaving with me, Dee [Dolores] says, ‘Now you children, you just behave yourself because when you’re out there people will look at you and they’ll say, look at those black children behaving badly.’

“And I thought, ‘that’s a terrible thing to say’. But what she was doing was giving them a message, which is that is the way people behave – so you’ve got to be extra on top of it because that is how people assume – will be about your blackness and not about you being kids. So it was a lesson for me to learn.”

What does Sheen remember about the actress when she first met her? “She was a vibrant young woman, full of purpose with an exciting charm who was quickly beloved by the students and faculty, and thank God she didn’t change. She invited us to be a part of her journey, and we welcomed her into ours ... an odyssey that takes children victimised by a zip code to heights of freedom for their minds and spirits – the Sheenway experience.”

But why have they kept in touch over decades and continents (Agutter taught at Sheenway for six years)? “Jenny holds a special place in our history,” said Sheen, “and I shall always love her for the gifts of hope that she fed into the dreams of our students.”

As Agutter remarks, neither the St Giles Trust nor Liberty Choir are “comfortable” charities – “It’s not dealing with children or ill health. It’s dealing with people that most people want to turn away from,” she said. “People that seem to have almost no hope in their lives, and we don’t want to recognise ‘no hope’.”

So why not pick a safer option for a charity to support? One with an easier sell? “Lots of people are doing that already and it also seems obvious to me that if you don’t take care of the most difficult elements of our society, we’ll never get anywhere because they’ll always remain.”

• You can help to support Liberty Choir’s work at: libertychoir.org/donate/ and St Giles Trust at: stgilestrust.org.uk/donate