Norrell himself is in his study, concentrating on a mirror which is focused on Lambeth. He is taking careful, detailed notes of the goings on in case they may be needed later. A record of actions, who acted how, faces, and so on. It may help him if things come back after this, to remember who acted according to orders and who did not.

He hears Childermass approaching but does not look up.

"Did I not tell you," he begins, "that I was not to be disturbed? You know very well how busy I am."

Not that Norrell is not always busy, but still.