With three new episodes lined up starting on New Year's Day, co-creator Steven Moffat says show's success came as surprise

An upmarket dominatrix, compromising pictures and royal connections: even the sharpest consulting detective will be kept on his toes when the BBC's much-anticipated Sherlock returns on New Year's Day.

With Benedict Cumberbatch and Martin Freeman ensconced once more in the magnificently wallpapered 221B Baker Street, the BBC will be hoping that A Scandal in Belgravia delivers a repeat of the ratings that made series one a hit.

Co-creator Steven Moffat said the show's success had been a surprise. "You couldn't anticipate something as big as this," said Moffat, who has penned this year's Doctor Who Christmas special.

"We thought that it would be well reviewed and sort of prestigious and smart … we didn't know it would be so whoppingly huge."

The first three 90-minute episodes had an average audience of more than 8 million as fans embraced the radical update of the detective's adventures.

Cumberbatch, who excels as the icy, eccentric detective, said: "In the world of streaming, Facebook, blogging and obviously Twitter you have an immediate audience feedback so you felt almost part of a live audience event. It was very strange."

The first of three new episodes, A Scandal in Belgravia, written by Moffat, features Lara Pulver as Irene Adler, the woman with a cameraphone full of secrets. Mark Gatiss's The Hound of the Baskervilles will follow, with The Reichenbach Fall, an update of The Final Problem, the last to air.

Gatiss said: "You get that confidence from the success of the first series and then just think well let's go for it. That was the thrill. The three most famous stories in one mini-series, three films, it doesn't get any better than those stories."

A Scandal in Belgravia sees Holmes trying to outwit the woman traditionally regarded as his only love interest. "What I love about it is it's so complicated. That's what's thrilling about it," Gatiss said. "It doesn't actually have to be as mundane as a love story – it's much more interesting than that."

Laura Pulver, who plays the woman who gets under Holmes's skin, said: "She is a very flawed, damaged, fearful woman and she has this wonderful mask … and then she meets this man and they see each other. It's like looking in the mirror."

Fans, however, are perhaps more likely to be rooting for the Holmes/Watson relationship, as the scripts and performance teasingly allude to.

"It's the only time ever that Sherlock Holmes and Doctor Watson have sat down 'no we're not gay,'" laughed Moffat.

Gatiss replied: "But everyone thinks they are – that's the fun of it."