20. Langdale And Porlock
When Mycroft and Sherlock attend a classified MI6 briefing with Lady Smallwood, Vivian Norbury and Sir Edwin, some codenames are dished out - Antarctica, Langdale, Porlock and Love. The first is probably a reference to Mycroft's cold nature and the last is later said to be Smallwood's codename.
The other two are names taken from the Conan Doyle canon, both of whom are informants and allies to Holmes. Langdale Pike appears in The Three Gables as a society gossip who provides Sherlock with some juicy rumours. Porlock, on the other hand, is a more interesting character as he is a double agent within Moriarty's ranks who feeds Holmes inside information about the criminal organisation. Given her later treachery, Porlock is presumably the codename of Vivian.