After the shock cliffhanger of Sherlock series three we knew the next instalment of the detective drama – whatever form it might take – was likely to address the apparent return of one Jim Moriarty.

Andrew Scott's Bafta-winning villain made an ambiguous appearance at the end of the episode when the closing credits were interrupted to show a gif of Moriarty taking over television screens across Britain and repeating the question "Did you miss me?" And when the titles finally ended there was a flash of the man himself, looking straight down the barrel of the camera.

But after apparently shooting himself through the head at the end of the previous series, could Moriarty really still be alive? Or is it some trick from beyond the grave that, according to BBC1, will see Benedict Cumberbatch's detective returning from "the briefest of exiles... to face one of his biggest mysteries yet"?

Whatever the answer, the Sherlock special – currently in pre-production and due to start shooting in January  will solve the mystery "completely", co-creator Mark Gatiss has assured me.

But when Gatiss, Cumberbatch and co then turn their attention to series four, will the one-off episode (which may or may not be shown at Christmas) be the last we'll see of one of the best-loved villains in TV history?

Gatiss's answer is as enigmatic and tantalising as you might expect: "Well, it's very hard to put a lid on Andrew Scott..."

Mark Gatiss: Next Sherlock is very special and tremendously exciting  but may not be at Christmas

Mark Gatiss supports the London Lesbian and Gay Switchboard, which is celebrating its 40th anniversary this year