It’s the biggest mystery of all — will we see Sherlock starring Benedict Cumberbatch and Martin Freeman back on our screens, and if so, when?

With the fourth series ending next Sunday – following last night’s explosive episode two - one well-placed BBC source says that whatever happens we’re not going to get more Sherlock next January.

But the BBC are “reasonably hopeful” of more Sherlock in two years’ time, according to the same source.

"Pre-production work would need to begin in the spring and we are not at that stage but the will to do more is there," said the source.

This view is echoed by showrunner Steven Moffat who tells this week’s Radio Times: “Right at this moment, we really don’t know about the future. Mark [Gatiss] and I have to have a long chat, possibly on a train, about what we’d actually do with another run. We love the show very much, and that means we don’t want to let go of it before we have to. All of us involved know that days like these will never come again and we’re in no hurry to see them over.

“At the same time, because we love Sherlock the way we do, we don’t want to keep it going past its natural term. There was a fairly long time we thought series four might not happen. Then Mark and I sat down, got very excited about some ideas, and in time pitched them to Sue [Vertue, the producer], then to Benedict and Martin. Only then, with  a clear view, did we all decide to get back to work. The same has to happen again — we have to be excited. We have to think of stories we can’t bear not to make.

“Everybody’s schedule is difficult. Benedict and Martin aren’t just two of the finest actors of their generation, they’re stars, in demand everywhere. Sherlock can only ever be a passion project. So give us a moment and Mark and I will go back to [Arthur Conan] Doyle and see what we’ve missed. The game may still be on.

“Unless, of course, everybody dies at the end of series four." 

What a tease...

So far fans had to wait at least a couple of years between each series. Clashing schedules of the leading men have been the biggest hurdle.

Series four was commissioned in October 2013 and announced in July 2014, but getting them all on set in late 2015 (as planned) proved impossible — filming didn’t actually start until April 2016.

This year finds Freeman in the zombie drama Cargo and supernatural thriller Ghost Stories. Next year he returns as Everett K Ross, this time in Black Panther.

Cumberbatch’s diary is just as crowded. This year he’ll be finishing The Current War, playing Thomas Edison, and Thor: Ragnarok. In 2018 there will be Avengers: Infinity War, Jungle Book (voicing Shere Khan) and How the Grinch Stole Christmas (as the Grinch). 

Whatever happens the popular drama is set for a dramatic showdown next Sunday after - spoiler alert - we discovered that Sherlock and Mycroft Holmes had a sinister sister called Euros played by Sian Brooke.

Not only does she seem to be in league with Sherlock's (dead?) adversary Moriarty - she also appeared to shoot Freeman's Watson at the close. 

Sherlock series 4 episode 3 The Final Problem aires on BBC1 on Sunday January 15

Sherlock series 4 episode 2 reviewed