Here is the link to the Christabel Chronicles
http://lesanctuaire.dreamhost.com/chronicles/contents.htmi have not read it yet but from the description it may be what SMac is describing.
Welcome to the world of the Christabel Chronicles, an epic alternate universe saga that attempts to answer the question: what if Buffy had met Angelus, the Scourge of Europe, before the curse returned his soul? How would the demon, unaffected by his soul's feelings, react to Buffy? What would young William the Bloody think of the Slayer from the future? What if Darla and Drusilla were already dead, but the Master was still around?
And what if there was no way home ...
Curious? Then journey to the world of Edwardian London, where you will find Demons, Slayers, Vampires, Watchers and all other manner of fascinating characters. However, before you venture into new realms, be warned that although these tales have various ratings, the Chronicles themselves are erotica and were written for adults only.