Yes, that's right. The Doctor has already appeared in Sherlock - but probably not the one you were expecting. Although technically 'appeared' might not be the right word. If you don't remember seeing Fifth Doctor (and father-in-law to David Tennant) Peter Davison in Sherlock then, don't worry, you didn't. In fact, his role was as the voiceover heard in the background in a scene from Series One episode The Great Game. When the gruesome assassin The Golem murders poor Professor Cairns at the astronomy centre, if you listen closely you can hear that it is Davison blathering on about planets and space. Well, to be fair, what else would the Doctor talk about?