I absaloutely love this (and the rest of course):
“I have only the one friend,” he said slowly, enunciating each word as though he was speaking to a small child, “because you’re my partner.”

The idea of everyone Sherlock knows having their own particular definition is a brilliant one.

(I don't see Hounds as being set at Christmas-time, so no quibbles from me about the timeline you use here.)