Answered in the comments:
"The man is now a commissionaire - it's his commissionaire's uniform he says is being mended. When Watson asks "and you were...", using the past tense, he is asking what the man did before he became a commissionaire. He says he was a sergeant, which makes him a retired sergeant in the sense that he isn't a sergeant any more (not in the sense that he is no longer working). You find this with military titles in old books." – user96060