Sarah Lancashire celebrated her 50th in October, but she doesn’t pay much attention to landmark birthdays, or ageing, for that matter.

“I live in a multi-generational extended family, and as my mother says, ‘You’re still young, it makes no difference’,” says the acclaimed actress.

“I think this country is terribly, horribly obsessed with age, and it really is just this country. If you’re still living and breathing at 50, then count your blessings!”

While Lancashire believes there are less roles as you get older, “it’s important to remember it’s not exclusive to acting”, she adds.

“It’s journalists, directors, teachers, lawyers. I don’t bang on about it, because it’s not the exclusive domain of actors. We’re just one tiny cog in a huge ageist wheel.”

Lancashire’s on the set of the third series of Last Tango In Halifax, a show she believes has been “a quietly groundbreaking piece”, in terms of facing ageism head on.

At the story’s centre are the lovestruck Alan and Celia, played by Derek Jacobi and Anne Reid, the former childhood sweethearts who reunited as septuagenarians and enjoyed a Christmas wedding in the last series.

But they’re not the sole focus of the plot. Elland-born writer Sally Wainwright’s script also follows Alan’s troubled daughter Gillian (Nicola Walker), and Celia’s uncompromising daughter Caroline (Lancashire) and their subsequent relationships.

“It’s a joy doing Tango. I’d do it ‘til the day I die, frankly,” enthuses Lancashire. “It’s so beautifully written, I’d turn up to read two lines a day just to be part of this piece.”

She’s keen not to divulge too much about her upcoming storyline, but does reveal that despite a tumultuous start, thanks to Caroline’s insecurities over her sexuality, she and partner Kate (Nina Sosanya), who’s now heavily pregnant through sperm donation, have found contentment.

The actress is also full of praise for writer Wainwright and said: “A lot of work goes into making good pieces of work.

“Sally’s sitting down writing the second series of Happy Valley now, and will be on that for God knows how many months. That’s a huge part of her life.”

Happy Valley, in which Lancashire starred as police sergeant Catherine Cawood, a woman still coming to terms with the rape and death of her daughter, while being hounded by psychopath Tommy (James Norton), was met with great acclaim when it aired in spring 2014, so it’s little surprise it was re-commissioned.

Much of the drama was filmed around Huddersfield, thanks to North Light Film Studios at Armitage Bridge, and they hope the cast and crew will be back.

“Right now, I’m not thinking about it, because I don’t know what the story is,” says Lancashire, who will also be appearing in the big-screen adaptation of Dad’s Army, due in 2016. “I have absolutely no idea what Sally has in store for me, but I’m bracing myself.”

She admits she owes a lot to her friend and collaborator. “Having two of Sally’s projects, it doesn’t get much better, does it?”

EXTRA TIME - LANCASHIRE’S DRAMATIC TURNS

Raquel, Coronation Street - From 1991 to 1996, she played the loveable, free-spirited Raquel in hit soap Corrie. There was a brief reappearance in 2000, when Raquel returned to ask Curly Watts for a divorce.

Ruth, Where The Heart Is - In the late Nineties, she played district nurse Ruth, opposite Pam Ferris, in this love and community-based drama filmed in Marsden and Slaithwaite. The show was a huge success but Lancashire reportedly decided to leave when she no longer felt challenged.

Yvonne, Clocking Off - In 2000, her success as Clocking Off textile worker Yvonne, followed by Carol, a woman who sets up a care home for abused children in Seeing Red, and barrister Ruth in Chambers, led to a two-year ‘golden handcuffs’ deal with ITV worth a reputed £1.3million.

Laura, Lark Rise To Candleford - From 2008 to 2011, she narrated as grown-up Laura in this well-loved period adaptation.

Miss Audrey, The Paradise - In 2012, she starred as a spinster in The Paradise, a period drama set in a department store, but had to leave halfway through the second series to film Last Tango In Halifax.

Last Tango In Halifax returns to BBC One on Sunday, December 28