As they take a seat side-by-side on the sofa, Pam Ferris and Jenny Agutter could not look more different to the nuns they play in the popular Fifties-set drama Call The Midwife.

For starters, they’re wearing make-up, which is a strict no-no for their screen alter egos, Sister Evangelina and Sister Julienne. The actresses admit that they enjoy being barefaced during filming.

"Oh, it’s lovely,” says Pam.

Jenny agrees. “It takes away from having to look in the mirror because you don’t really want to,” she adds, laughing.

Not that she’s disappointed by what she sees on screen.

“I’m proud of every line on my face,” says the 60-year-old star, to which Pam, 65, adds: “Exactly – I’ve earned all of those!”

Instead of a nun’s robes, today Jenny’s opted for a smart, red skirt suit and Pam a long dark dress – and much to their delight, they are free of their wimples.

“Urgh, I have a hate-hate relationship with my wimple, me no love,” says Pam.

“It puts you in a very separate world,” notes Jenny, who in the Sixties became a star in The Railway Children film, filmed on the Keighley & Worth Valley Railway. She’s worked steadily since, with roles in shows like Spooks and The Invisibles.

“Sometimes you can’t actually hear what people are saying, and after a while you give up saying, ‘What?’, and just nod and smile.” Pam laughs. “This is the theory,” she notes. “Nuns can’t solve your problems but they can certainly pretend to listen!”

Wimples are at least warm, which is beneficial when filming in winter. Unfortunately, the Call The Midwife Christmas Special was shot during the June heatwave.

“The thing to remember all the time is cold, cold, cold,” says Jenny. “When one’s cold and pretending to be hot, that’s difficult, but when it’s hot you forget that you’ve got to be tightened up – physically, you’ve got to see that.”

The pair have just watched the Christmas Special for the first time. It will be screened on Christmas Day, with series three following in the New Year.

It’s the second time the show has done a seasonal special, and it’s not a calm Christmas for the Nonnatus House team, after an unexploded bomb, a relic from the Second World War, is discovered in Poplar.

“I tell you what I thought with this storyline,” says Pam. “How naive it is to think the war ended in 1945.”

But it’s not all explosives. There are, of course, many births at the nursing convent, as well as an emotional storyline involving Dr Turner’s son, Timothy, as the doctor prepares to marry Shelagh – formerly known as Sister Bernadette.

Pam, who’s also enjoyed a long career with highlights including Rosemary & Thyme, The Darling Buds Of May and Harry Potter And The Prisoner Of Azkaban, admits she cried as she watched the special – not what you expect to hear from the actress who played the formidable Trunchbull in Matilda.

“It’s embarrassing if you’re in the damn thing!” she says.

Her husband is no better. “I’ll watch it with him on Christmas Day and I’ll have the tissues ready – he’s a bit of a weeper.”

Pam is having a very untraditional Yuletide this year.

“I try to steer clear of Christ-mas rituals,” she says, admitting that their Christmas dinner will be steak and kidney pie.

Is it the spiritual element, then, that makes Call The Midwife so perfect for Christmas Day viewing? “I can say there’s something rich about people who’ve made such a strong commitment to helping others – I find it very moving,” says Pam.

Jenny agrees: “It’s their dedication that’s extraordinary.”

Call The Midwife Christmas Special is on BBC1 on Christmas Day.