After Jonathan leaves for the Peninsula, Mr. Norrell realizes that he is missing a book that wasn't on Jonathan's list. He guesses that Jonathan took it home with him one day to study and so Norrell sends Childermass to Jonathan and Arabella's home to retrieve it. Childermass is obliged to visit several times when Norrell suspects that a missing book may be at Jonathan's. Arabella is initially cold, hostile, and suspicious towards Childermass; Childermass is initially dismissive towards Arabella. But the sexual tension should build and build until, against the better sense of both of them, it positively explodes.

It could be hatesex, it could be genuine desire and feelings, or it could be simple lust. I leave that up to the imagination of the writer. Just make it hot. ;)