Back in June, Sherlock star Martin Freeman let slip that the next time he and Benedict Cumberbatch would unite to film the detective drama it would be for a Christmas special. But when the official announcement came that filming on a one-off episode – designed to bridge the gap between series three and four – was to begin in January, it was described simply as "a Special", with no reference to festivities or seasonal scheduling.

Now, Sherlock co-creator Mark Gatiss has cemented the ambiguity (if that makes any sense), saying that, while pre-production on the episode has already begun – and while the story would work in a Yuletide slot – it's still not clear whether it will be the 2015 Christmas gift fans are wishing for.

“It will be finished, of course, by the end of spring," Gatiss tells me, "[but] when they show it is entirely down to the schedulers. It would fit as a Christmas special but there’s no guarantee of that – they used to show the Mike Yarwood [Christmas] Show in the summertime.”

The good news, according to Gatiss,is that "it’s a very special special, and we’re all tremendously excited about it".

And if that wasn't quite the level of detail you were hoping for, he's quick to point out that, given the now traditional gathering of fans at Sherlock filming locations, more information is likely to become available for those who want it in the not-too-distant future.

"It’s pretty close to shooting now," says Gatiss, who will return as Sherlock's brother, Whitehall grandee Mycroft. "It won’t be long before people see enough to get their teeth into...”

But what exactly can they expect? 

Well, one way or another the episode will address that series three cliffhanger – the apparent return from the dead of Andrew Scott's supervillain Moriarty. Is he really still alive after apparently shooting himself through the back of the head? Or was the glorified gif that appeared on screens across the country during the closing moments just a trick being played from beyond the grave?

The Sherlock special should answer those questions – but will it be Christmassy? If, as usual, Gatiss and Moffat borrow elements from Doyle's original Sherlock Holmes stories, and if they want to create a seasonal vibe, there's one in particular they'll be revisiting.

The Adventure of the Blue Carbuncle is a fun and atmospheric tale of a stolen gemstone and a Christmas goose –and one that Gatiss once told me he adores and would love to adapt.

When I asked him about it this time around he was staying predictably tight-lipped but if he and Moffat do end up delivering a proper seasonal special, shown in December, then I like to think it's a helping of Christmas goose those fans will be sinking their teeth into...

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