It felt like Chapter One of a YA novel, after being sent back down Sebastian develops hitherto unnatural spine and returns above to burn down the tower blocks and upend the social order. At the end I just thought 'meh'. I felt there was insufficient work put in to explaining the world we were in and how everyone worked within it. Even Hammersmith at the end is an enlightened being talking with a non-enlightened being within the system that the author has created and so doesn't explain how the world works in any way that made sense to me. So the story in the end is 'boy finds sack, boy loses sack, boy goes up stairs, boy goes down stairs. With mucus.'