Childermass inclines his head slightly. In truth, he is Norrell's servant. As well as his businessman, in some senses of the word. And his driver, at others. And his guard dog. In this situation he's affecting something of the latter state of being.

"And what about it?" he inquires, not particularly politely, but then again it's not his business to be polite. It's his business to keep Mr Norrell out of trouble, and he'd be doing a poor job of that indeed if he let just anyone asking about for Norrell into the house. (And he'd certainly get an earful about it later, for that matter.)