I am ok with your character having some knowledge of the Neath, but be careful not to let him know too much. Most of the citizens of the Neath do not know many of the mysteries of the place, and most of the surface dismisses the stories from the neath as tall tales. I would say that he can know things that an average londoner would know, but have some misinformation mixed in, seeing as he has has no real way to verify any of the stories he has heard.

Also, Malkin, you don't need to apologize about your character acting in character. It is fine and dandy, I would be skeptical about touching my thumb to a magical sigil thing too.