The daughter of a teacher mother and a businessman father, Jennifer had a happy upbringing (with her younger sister Eleanor) in the countryside near Great Malvern in Worcestershire. ‘I go back as often as I can – it’s very grounding.’

She attended an independent girls’ school where she was a diligent student who always had her nose in a book and ‘never even considered acting – it’s the last thing anyone would suggest when you’re as shy as I was’.

It was only when studying for her music GCSE – ‘realising I was rubbish at it’ – that the thought of switching to drama occurred to her. ‘From that first [acting] lesson, I felt a rush; my shyness vanished. It took me a while to say out loud, “I want to be an actress.” When I told my parents they were a bit shocked, but they supported me from the off.’

She studied English literature at the University of East Anglia in Norwich. ‘I could have applied to drama school, but I knew I needed to do a bit of growing up and figuring out who I was first. And I’ve always loved the academic side of things. I’ve re-read Jane EyreWuthering Heights and Madame Bovary.’

After graduating, she was accepted at the prestigious Lamda, and then landed the role of Elizabeth Bennet in a stage adaptation of Pride and Prejudice at London’s Regent’s Park Open Air Theatre, for which she was nominated Outstanding Newcomer at the Evening Standard Theatre Awards.

‘It was amazing to get that job just before leaving Lamda, and as a huge Jane Austen fan, to boot. It was when I was standing next to Joanna Lumley at the awards ceremony that it occurred to me that this acting thing might really happen for me.’

Since then, she has barely stopped; but until Call the Midwife came along, her major roles were on stage, mostly with the RSC. ‘Shakespeare is something I will always go back to: the language is all about human emotion and it is much simpler than it seems.

‘At first it was a big transition to television,’ she recalls. ‘Most people would say stage is scarier, but for me screen was, until I got used to it – it requires such different skills; there are so many people on set and behind the camera, and you have to draw viewers in. What works on stage won’t necessarily be enticing on screen.’

Luckily, the cast of Call the Midwife made sure that Jennifer felt comfortable. ‘They are the nicest group of people. Once I got past the star-struck moment of meeting Linda Bassett and Jenny Agutter, two of my favourite actors [who play Nurse Phyllis Crane and Sister Julienne, respectively], I relaxed, because everyone was so welcoming.

Working with your heroes – a cast of legends – could be quite daunting, but they’re my friends now. I love being part of a team with everyone interested in how our characters relate to one another. And, of course, the babies create a wonderful atmosphere on set – except when one of them starts to cry at the wrong moment.’

The cast socialise outside work, says Jennifer – earlier this year, they turned up en masse to see Charlotte Ritchie [who plays Nurse Barbara] in The Philanthropist on the West End stage.

On set, ‘We talk about anything and everything. And we’re usually eating – sandwiches, biscuits and croissants with jam. We eat like people did in the 60s! There’s no dieting on our set. The other day we were filming a scene at the dinner table, and the food was so good that after the cameras stopped rolling we all stayed and devoured it. The director and crew left us to it.’

She also confesses to a retail addiction, revealing that she and Victoria Yeates (Sister Winifred) have the same taste ‘and do a fair amount of online clothes shopping between takes’. Zara, Topshop and Urban Outfitters are favourites. ‘I love fashion too much. It’s something I have to curb. I don’t ever buy the thing that’s pretty, though; I’m that person who chooses whatever is odd. Very often I dress like a boy.’

‘I don’t ever buy the thing that’s pretty; I’m that person who chooses whatever is odd. Very often I dress like a boy,’ says Jennifer

Indeed, today she is in jeans and a grey shirt, a look that complements her androgynous hairdo. Taking on the role of Val meant chopping off her long tresses in favour of a no-nonsense crop. ‘I was happy to do it; the new do felt like a way into my character. To be honest, I was so happy to have the job I would have let them shave my head.’

Jennifer drew praise for donating her locks to the Little Princess Trust which makes wigs for children with cancer. ‘That was a no-brainer. There was so much coming off it was wonderful it could go to someone who will value it more than I ever did.

‘The way they backcomb my hair for Val is completely different to how I wear it, so I have that line where she ends and I begin.’

JENNIFER’S GEMS

On TV I never got into Mad Men first time around, but I am obsessed with it now.

Keep fit Hot yoga – I love how you sweat every toxin out of your body.

Most memorable trip I spent a month in China with the RSC. That was incredible – performing in Beijing, Shanghai and Hong Kong.

In three words, you’re… Outgoing, shy – at the same time – ambitious.

Literary heroine Hermione from Harry Potter – I absolutely love her.

Last meal on earth I can never say no to a pizza.

Perfect day Regent’s Park in the sunshine – and reading, if I’m by myself.

This distinction between herself and her character is clearly important to her, although there is, it seems, a fair bit of overlap. ‘I aspire to be as brave as Val. She is feisty and speaks her mind. She and the other midwives are very modern in that respect – and in the way that midwifery is their calling which they put before everything, including finding a man, which I think makes them ahead of their time.’

Jennifer is equally unafraid to make her voice heard (as evidenced on her Twitter feed) when it comes to causes that are important to her, such as voting. ‘The world is so bad right now that I hope people will realise they have to get involved and stand up for things. For me, the most important issue is our NHS. It is a glorious thing that we should be proud of and fight for.’

This is spoken like a true nurse-midwife. ‘I wish! I could never be a midwife, much as I would love to. Valerie is infinitely more practical than me. To have all that capability is something I can’t even imagine.’

How does she hold it together on set, amid all the death and tragedy? ‘I often want to have a little cry, but I remind myself that my character is not the one undergoing the trauma. She is a professional and it is really important that I leave the crying to the ones who are meant to be sad.’

And while it may not be as important as bringing new life into the world, I have to know, will single-girl Valerie find love in season seven? After all, Barbara has married handsome vicar Tom [Jack Ashton], and Trixie [Helen George] now has a fella, so surely it is Val’s turn?

‘Perhaps,’ Jennifer says coyly. ‘But the great thing about the show is that while the women do have relationships, they are never defined by them. A love interest is merely a healthy addition to their lives. But that said, everyone needs a bit of romance…’