How do you view Call the Midwife, BBC1’s hit adaptation of Jennifer Worth’s memoirs about a group of midwives and nuns administering to London’s East End in the 1950s and early 60s? As a cosy, saccharine drama about a time when we all pulled together, communal balm now for the current divisions of Brexit Britain? Or as a deftly written, unflinching and emotionally compelling recollection of the difficult birth-pangs of the NHS - of dedicated women doing their best in often desperate situations?

In America, where the show is shown on the PBS network, they are in no doubt. The LA Times, in the midst of the wrangling over Obama-care, described Call the Midwife as “a timely Valentine to socialised medicine”, while another paper said it was “the Breaking Bad of midwifery drama” – both descriptions delighting Heidi Thomas, who writes the whole show single-handedly in the upstairs bedroom of her house in Saffron Walden.

“In America, midwifery is seen as quite radical”, is how Thomas explains the difference in the cultural outlook. “Over here 82 per cent of babies are delivered by midwives, but in America only eight per cent of babies are – and they’re often from families who are Christian or follow an alternative hippy way of life.”

It’s more than that, of course, and Thomas’s scripts skilfully draw out the human stories from the very tough circumstances that new mothers faced in the London docklands – from botched illegal abortions and domestic violence to ever present threat of TB, polio and other now largely conquered childhood diseases. And in the most recent series, the resident GP, Dr Patrick Turner (played by Stephen McGann, who is married to Heidi Thomas) discovered to his horror that the morning sickness pill that he had been prescribing, thalidomide, was causing the birth defects he has been witnessing.

This year’s Christmas special sees some of the nuns and midwives answering an SOS and heading to South Africa to deliver polio vaccinations to a tiny mission hospital located in the scrub outside Cape Town. It’s 1961, the year after the Sharpeville Massacre, in which South African police fired on a crowd of black protesters, killing 69 of them, and the year before Nelson Mandela was arrested and sent to Robben Island.

“They are not political people and this is not a political piece”, says Jenny Agutter, who plays Sister Julienne, when I join her in her trailer on Call the Midwife’s permanent home on the western fringes of the M25 in Surrey. “But you can't help but be aware of the things that are going round, and the restrictions.

“Apartheid is not shoved in there, but it's referenced by what's happening to the black community - a lot of heavy-handed things went on in the Sixties. I went to the slavery museum in Cape Town and there's a big exhibition on apartheid – one goes right into the other – and the shocking thing is that we could all have sat by for all those years, where ordinary human rights are abused. It's going to be hundreds of years before they can get over that history of abuse.”

For Agutter, who turns 64 just five days before the Christmas special, her character’s imagined ship’s voyage to South Africa brought echoes of her own real childhood experiences. “We don't really see the journey [in the show] but it would have been on a steam boat and I went by boat as a child to Singapore and, because the Suez Canal was closed at that time, we had to go to South Africa”, she says. “And it would have been a similar boat they took, I remember it well… I remember stupid things like the smell of it, the food we had on board.”

Agutter’s father was a former army officer and entertainment organiser, and her childhood homes included Singapore, Cyprus and Malayia, inculcating a lifelong wanderlust that, this Christmas, sees her making her first visit to New Zealand. The new series of Call the Midwife, which begins in January, will witness the staff at Nonnatus House reaching 1962, and the issues faced in coming episodes include the Cuban Missile Crisis, the growth of the East End gangs and the emergence of the Krays, and the greater availability of the contraceptive pill.

“Contraception is something that's important because Julienne sees the terrible strains that that has had on women in the past… not being able to control their lives”, says Agutter. “But she's very worried by the recreational side of it, as to what that means or whether that's to be encouraged or not.

“You automatically assume that the Sixties brought so many good things, which of course it did... it opened up a lot of things... but it also pinpointed a lot of social problems which are as big today as they were then. People are still bigoted, there's immigration, and so on. Last year I thought they were extraordinary stories, and we had that over-riding Thalidomide storyline. Heidi does get a lot of energy feeling that there are real circumstances she can draw on.”

The 1960s were also the decade in which Agutter came of age herself, as a young woman and as an actress – including her breakthrough role in The Railway Children. Is it slightly strange, I wonder, for the drama to be entering into a period that is part of her own experience?

“I remember being at boarding school with the big freeze in '63, but that's the next year”, she says. “You have to keep telling yourself that my memories as a child are quite different than Sister Julienne's feeling about the time, she has her own experiences – including two world wars.

“It's only as you get older that time begins to draw in on you and you realise the impact that history has – we're still caught up in things that happened in the Sixties. It's terrifying when I hear politicians talking about stepping back in time… back to the good old days.”

The Railway Children – the 1968 BBC adaptation of the ES Nesbitt classic and the subsequent 1970 film – may no longer get shown on TV every Christmas, Agutter ruefully observes, but two of her other movies have recently received positive reappraisal: Walkabout, Nic Roeg’s 1971 classic in which Agutter’s adolescent girl and her younger brother, abandoned in the Australian outback by their suicidal father, are befriended by an Aboriginal boy; and An American Werewolf in London, John Landis’s 1981 landmark comedy-horror, in which Agutter plays a nurse in love with the eponymous man-beast.

“American Werewolf has come again... they're having a screening of it because they've got a new super-duper cleaned up version coming out”, she says, referring to the restored Blu-ray edition that was released in September. “And Walkabout has been on at a huge open-air thing at Somerset House [part of Film4’s annual outdoors cinema event in London].”

“I have such vivid memories of making Walkabout. The desert... the sleeping out... I was quite a young 16-year-old and my mother came out there, but then I was on my own, just this group of people travelling around... that sense of being slowly absorbed more and more by that desert space, and very touched by it. I found it very powerful.

“David [David Gulpilil, the indigenous Australian actor who plays the character called Black Boy in Walkabout] would tell his own dream-time stories that I never really fully understood – we spoke not only different languages but we were from completely different cultures and we were only 16 and no idea who the other person was.

“I've met him since and I've heard through friends who've come across him since that he's made a lot of changes in his own life. But back then we were 16 and poles apart.”

For a certain generation, Agutter was, or still is, a sex symbol. “Yes, she laughs. “I sadly get people saying ‘my grandfather really liked you as such and such’… or 'My dad used to think you were lovely'.”

For now, however, it’s back on with Sister Julienne’s wimple, filling the water bowl for Sophie, the wire-haired dachshund who accompanies her on set, and who has been growling at me suspiciously, intermittently throughout the interview. What’s next, after her return from New Zealand?

“I'd love to get back to America and make some more Avengers”, she says, referring to her role as Councilwoman Hawley in the blockbuster Captain America: The Winter Soldier. “It's just such a lovely contrast to playing a nun. I like the idea of going kicking ass.”