“There is a clue everybody’s missed,” he says tantalisingly. “So many people theorising about Sherlock’s death online – and they missed it! We’ve worked out how Sherlock survives, and actually shot part of what really happened. It all makes sense.” In this, he argues, he and co-writer Mark Gatiss have gone one better than Doyle. “He cheated outrageously. He has Watson deduce that Holmes fell off a waterfall. But there was no body. And it only means one thing in a detective show when there’s no body.” That the victim survived. So you set yourself the test of killing off Sherlock, putting his corpse in plain view and then bringing him back from the dead to watch his own funeral? “Yes. We had to have Holmes dying in Watson’s arms – and get away with that, which we have.” But how? Moffat sips his tea silently.