His mouth was silently working as though he were chewing on a slice of lemon, but that was his habitual expression, so Albus did not take it to heart. Best Severus and Albus line I have ever read, I think!
oh, well done! This story is brilliant and inventive. Just like Dumbledore to botch up something in that manner, and just like Filch to know the reality of it. Head of school and head of household being the same thing, hah!
You managed to write this relationship in a prefectly Snape manner, which is difficult. Fantastic! Filch was a laugh, as well - I can see something like this based on his behaviour around Umbridge.
Great writing structure (the quotes were genius), the style is excellent and I am all-round impressed.