And the answer was B: Book 6: Future Shock
In that book Emma Earl is transported to the future, where the evil Isis returns more powerful than ever and cripples Emma, aka the Scarlet Knight. Emma's daughter Louise then takes up the mantle to save the day. I really liked the story (which in itself is based on the Batman Knightfall saga of the mid-90s) and so decided to essentially rewrite it as a standalone book that became The Night's Legacy.
Christopher Dilloway woke up early and answered first so he got the bonus prize plus 5 points and Cindy 3 which makes it Cindy 23, Chris 11, Everyone Else 0