When Mr. Mason comes to 221b Baker Street to see Holmes, he states his opinion that his employer, Sir Robert Norberton, has "gone mad." Sherlock replies "This is Baker Street, not Harley Street." Harley Street has long been the address of many private clinics and doctors' surgeries.
Jude Law (Joe Barnes) later played Doctor Watson in Sherlock Holmes (2009), Sherlock Holmes: A Game of Shadows (2011) and Sherlock Holmes 3.
Ten years previously Robin Ellis and Jeremy Brett starred together in the Granada TV adaptation of The Good Soldier so this must have been a reunion for them.