The Mitford sisters were Outrageous – the title of new TV biopic is justified, say cast

“They were definitely women who didn’t want to just do the debutante balls,” adds the actor most familiar from Bridgerton. “They wanted to work and to exercise their brains.”

However, apart from Nancy, who made a bit of money from writing, “they either had to rely on rich husbands or allowances from Papa. And if you’re not giving people a place in society where they feel valued, they’re going to turn to places where they feel that they’re contributing. For some of these sisters, that ended up in problematic areas. When their voices weren’t being heard, that built up their anger and frustration. If we don’t listen to the voiceless, they’ll just scream louder.”

These self-reliant women feel decades ahead of their time. “The Mitfords were modern women,” agrees Carter, “they were really progressive. They always asked, ‘Why can’t I do that?’”

That independent-mindedness, Carter believes, was down to the Mitfords’ “fierce determination. Although their father said that women shouldn’t be at school he let them read every book in his library. So they were extremely resourceful.”

At its heart, Outrageous charts the relationship between Nancy and Diana, which broke down after Diana’s marriage to Oswald Mosley. “Nancy had to take a stand,” Carter explains. “We’ve all probably had that moment when you have a sibling who’s going out with someone who makes you go, ‘Aargh!’ But this was on a global scale. And that really drove a wedge between Nancy and Diana.”

Beyond that, “Outrageous is looking at these six sisters and wondering how long a family will hold together. And at what point do you have to confront hard truths and face your differences? That’s relevant today, isn’t it? We probably all have a family member who politically is in a different place. How do we deal with that?”

The actor says that Nancy’s feelings towards her parents in no way reflects her loving bond with her own parents, Jim Carter and Imelda Staunton, with whom she is currently starring in a West End production of Mrs Warren’s Profession – “It’s a very different relationship”. That also extended to her feelings towards her on-screen mother – “but it did help that I knew Anna Chancellor and that made me feel very calm. She’s an amazing woman, and so I felt very relaxed in her company. She became like a mother to all of us.”

How does Carter think viewers will respond to the show? Now more than ever, she stresses, “We have to talk and listen to each other.”

Joanna Vanderham confesses the outfits she wears as Diana Mitford did, “half the work for me. I had these incredible power shoulders, I felt like I could take on the world. I have a scene where I’m telling Churchill how to run the country – and that’s all because I have my power shoulders on! That’s the power of a good dress.”

The 34-year-old Scottish actor, who has also taken leading roles in The Go-Between and The Paradise, relished being able to dive into an enormous pool of research about the Mitfords. “One of the things I enjoyed was looking at old photographs of Diana because she was the socialite of the time. She was in the tabloids before anyone else.

“There’s a whole wealth of imagery. Diana had this unique thing that she did with her eyelids. She struck this pose where her eyelids were at half-mast. Apparently, she hardly moved her face when she talked or posed for photographs because she didn’t want to give herself wrinkles.

“So, I try throughout the show to have my eyelids half-closed. And there’s only one way you can hold your head and see if your eyes are half-closed – you have to look down your nose at people. That just brought her to life for me.”

All of the Mitford sisters’ stories are extraordinary, but Diana’s stands out. She was initially part of the Bright Young Things, a group of Bohemian socialites in 1920s London about whom Evelyn Waugh wrote in his novel Vile Bodies.

But that all changed when she left her wealthy husband Bryan Guinness for Oswald Mosley. Their marriage in 1936 took place at the home of the Nazi propaganda minister, Joseph Goebbels, with Adolf Hitler as a guest of honour.

Diana’s involvement with fascism resulted in three years’ internment at Holloway Prison during the Second World War. Vanderham explains, “Nancy told on Diana. She informed Churchill that Diana was a threat to national security and had her imprisoned. Who does that to their own sister? The Mitfords were mad!”

The actor, who has two sisters and a brother, says she was able to draw on her own family dynamic, saying of her siblings: “They’re the first true friendships that you ever have. They’ve been through things with you that no one else has. It’s a really special relationship that I treasure. Thankfully, I get on very well with my family. Unfortunately, some of the Mitfords stopped talking to each other entirely.”

Diana never changed her tune. Her interview on Desert Island Discs in 1989 was condemned because she denied the Holocaust and expressed her unrepentant admiration for Hitler.

Her story is just one element of a real-life drama that is barely believable. Vanderham observes, “I feel that people will go, ‘That wouldn’t have happened.’ Then they’ll Google it and realise it did. The Mitfords were outrageous. The title is absolutely justified.”

The latest issue of Radio Times is out now – subscribe here.

Outrageous will air on U&DRAMA and U from Thursday 19th June.

Check out more of our Drama coverage or visit our TV Guide and Streaming Guide to find out what's on. For more from the biggest stars in TV, listen to The Radio Times Podcast.