The Sherlock writer’s episode, starring Daniel Mays and Emma Cunniffe, was originally planned to air as Episode 4 of the show’s sixth series, but was shifted to the second half of the series in January.

Speaking at the Kapow convention in London at the weekend, DigitalSpy report that Gatiss commented: “It was shot first, and it’s now in the second half of the season, so it’ll be well over a year between shooting and it being on [television]. We had to drop the last scene, because it no longer fits. I had to write a new scene to go on the end, to make the second half make sense.”

Discussing the episode’s story, he teased: “It’s my first modern-day [episode]. It’s set in a tower block and it was filmed in Bristol. It’s essentially about a little boy with OCD. His parents are despairing about him, because he’s frightened of everything, and [then] The Doctor turns up and tells him he’s right!”