On New Year’s Day, Sherlock – the most watched British television drama series since modern viewing figure records began (in 2001) – returns, and 20 per cent of the UK population will tune in. For one night only, Sherlock Holmes and John Watson, brought into the modern day in 2010 by Benedict Cumberbatch and Martin Freeman, will step back in time to the Victorian era of Sir Arthur Conan Doyle’s novels.

According to Amanda Abbington, who plays Mary, the mysterious ex-assassin who took aim at John Watson’s heart, viewers will be dazzled.

“The fact that we were doing a Christmas special was already exciting. I had never done one before. When Mark Gatiss and Steven Moffat [co-creators of the Sherlock reboot] told us the premise, we were all blown away,” she says. “We are going back to Conan Doyle’s vision. There is a darkness to his stories. I’m fascinated by the history of that time, those extremes of poverty and wealth. So I love that we have done a one-off in the original format.”

They sit silently for hours to watch how it is filmed. We go over and talk with them and they get excited

Since joining the cast in 2013, Abbington has had a close-up view of the intensity around the show’s stars, Benedict Cumberbatch and Martin Freeman (her real-life partner since 2000). Obsessive fans attend every location shoot.

“The people who come to watch us filming are sweethearts – 99 per cent are well behaved and lovely,” she says. “They are often really young girls, and they sit silently for hours to watch how it is filmed. We go over and talk with them and they get excited. When Martin and Ben talk with them – I hesitate to compare it to Beatlemania but there is that fanatical element.”

The importance of the show to this younger female audience is something Abbington takes very seriously.

“I am fully aware that there are young, vulnerable girls that watch the show. I know what that is like because I was young and vulnerable once. I was bullied a lot as a kid. So when young girls come to me and say they are having trouble at home, or being bullied at school, a few words of encouragement can go a long way to making them feel less alone.”

In recent years Abbington’s profile has increased thanks to her role as Miss Mardle in Mr Selfridge, which returns in the new year, starring opposite Jason Isaacs in Case Histories and, most recently, playing a central character in the pre-watershed police show Cuffs.

“The other day a police officer in London wound down his window and said, ‘Excuse me madam, I just wanted to tell you we are loving Cuffs and all the boys at the station say it is incredibly accurate.’ You can’t get bigger validation than that!”

With bigger and better roles comes more pressure. Abbington talks passionately about her desire to keep her children out of the limelight – something her Twitter followers help to police. She has had years to adapt but others – in particular Benedict Cumberbatch – had ‘Beatlemania’ status thrust upon them. Sherlock propelled him from respected actor to globally beloved megastar and pin-up, and major life events – marriage and fatherhood – have intensified pressure in the last 12 months.

“To do all that in such a short space of time must be incredibly overwhelming. Martin and I did it over 15 years. We were together for a long time before having children,” she says. “I spoke to Ben at the Hamlet first night, and it was lovely. He is over the moon that he has a little boy. He is riding that wave of being a famous actor and loving every minute of it. And why wouldn’t you? But he is very private.

“You have to try to keep an amount of anonymity and not parade your children around. It is not our kids’ fault their parents are actors. We try to keep ours out of the limelight, and I have no doubt that Ben will do the same with Christopher.”

During the 2015 election campaign, Freeman made a campaign video for the Labour Party and Abbington spoke out in support. The reaction was intense.

“Martin particularly stuck his head above the parapet and we got shot down in flames. It was like a witch hunt, how much more stuff can you throw at us?” she recalls. “He was very badly burnt by that. I don’t know how politicians do it. So we quietly champion now because that side of it isn’t worth getting shit for, it really isn’t.

“But I thought what he did was brilliant, he was very honest about what he believes. I like what Jeremy Corbyn stands for, I like that he wants to help people. Martin said to me the other day, ‘You are one of the few people I know who gets more militant as they get older.’”

We got shot down in flames. It was like a witch hunt, how much more stuff can you throw at us?

After a quiet Christmas, Abbington plans to take a few months out to be at home. She talks about being a “music nut”, passing on the love of music she inherited from her father to her children.

“We have a very big wall of vinyl that we get out and dance around with the kids. We had music in our house from when I was tiny. Now we try to school our kids in what is good,” she says. The Unthanks and Larkin Poe are current favourites.

“At some point I have to take my daughter to see Taylor Swift. It is so sweet, her first dip into someone she really likes. It is really cool seeing someone very young being a fan of somebody. And I’m very glad it’s Taylor Swift because I think she is decent, a force for good.”

In the new year Abbington will complete work on Another Mother’s Son, a film set in Nazi-occupied Jersey, and is in discussion about a four-part BBC comedy. First, though, she will join up with the Sherlock gang – including their hardcore fans – to film three more feature-length episodes, starting in April.

“Steven Moffat says it is all going to hell, which sounds great to me,” she says. “I can’t wait. I know what is going to happen. I know what a rollercoaster it is and how dark it is going to get. They have done it again. Sherlock fans are going to absolutely love it.”

Sherlock: The Abominable Bride airs New Year’s Day at 9pm on BBC One

Amanda Abbington

Amanda Abbington: “Sherlock is like Beatlemania”