S4/5 fic where they find out that Spike's Demon was one of the First Slayers followers

Hey I'm hoping someone can help me here.

I read it ages ago and have been checking every sight I can think of but had no joy.

I can't remember much but I'll give what I know.

I'm pretty sure it is set in S4 as a few key scenes are set in Giles' apartment.

Buffy and Spike realize they belong together.

Through the course of the story they realize the reason Spike was so drawn to her was because his demon swore to fight on the side of the First Slayer. They also find out that Angelus' demon fought against her which is why he had such a problem with Buffy.

Buffy ends up hysterical for some reason that Spike can't figure out and he has to take her to Giles, not knowing what else to do. It turns out she thinks the only reason he wants her is because of the First Slayer.

I think it's Giles who figures it out and tells him to stop being an ass and prove it to her.

Thanks in advance.

Forgot to Add - It isn't Blood Warms the Heart although that's a good story too.

Is it this one?

The Better To Bite You With by Joyful Dayz

Summary: Season 5: Buffy has discovered that The Hellgod Glorificus captured and tortured Spike in an attempt to make him tell her where the key was. This is the "fang fic" that tried to barge its way into "A New Day".

Yeah, that's it.

Thanks