Oh, this is amazing! I was hooked from the descriptions at the start, completely there with Slughorn, who made me smile every bit as much as the books. He's so lovely with his old-fashioned weaknesses and habits.
I love the clues you give the reader, and Horace's thoughts as he works them out, especially that he wasn't completely sure until the potion. It's really nice to see that he actually cared, just the little glimpses you give us, even if (being Slughorn) he didn't do anything about it. Oh, and the mention of Halifax was especially cool for me, as that's where my family comes from.
This is my favourite bit, though, because yes. Absolutely the attitude Severus would have grown up with. Horace knew the sort of people he came from. You didn't just go and throw away a perfectly serviceable pair of boots, even if you were leaving the rest of your life behind.
What a wonderful gift. Thank you so much, anonymous Santa!