I think I read through that beginning dialogue ten times trying to decide if that was Martin, and why Mycroft would refer to him in the third person (my conclusion being faulty listeners; he might possibly have been calling him Martin and 'didn't' was the start of a new line. Or it was a quote but the first seems more likely) and then the entire chapter again because I could have sworn they referred to the assumed Sherlock as being dark haired...but only finding Mycroft's hair described.

So...either that is indeed Martin...or Sherlock is acting like his brother to throw them off (possibly to convince them he's more harmless than he is). Or it's Sherlock and you want all your readers to think it's Martin.

Whatever the truth, I've probably spent way too much time pondering it for this late at night/early in the morning. And I'm beyond thrilled that there is a sequel. Intriguing and ominous start.