"The reading room in two hours, then," he says, never having learned how to accept a thank-you no matter how many people tried to teach him, and melts into Knockturn. Harried from behind, that's the ticket, not afraid, just too pressed forward with business and maybe an irritable boss to have time to bother for anyone in front of you, except to avoid bumping into them and getting your pocket picked, just fade into the cobblestones and the grimy, sooted, ill-lit walls--but not too far, let them see you wandering about, like any good-faith runner protecting the identity of anonymous sellers...
This is probably all overkill, but 'probably' has killed the lax before, and in any case, it does a bloke no harm to be seen putting in the effort to uphold professional ethics, especially in professions where the ethics are so... specialized. And he might run into old contacts, and besides, one ought to keep in practice.