The BBC, ladies and gentlemen.<-- Am I the only one that thinks, if there is ever a special where this happens, that Peter NEEDS to play old Sherlock!? <--- Yes. But I would love the Doctor to meet Sherlock
Maybe Clara actually was in the same universe as Sherlock and Mycroft at one point and she was their nanny and left that behind. Mycroft keeps it because she was the only one who really loved him for him.