I have read and enjoyed other works by Green. This book I have read. What was the most difficult for me was the speech styles. "The chain of events opened a door, and now Something from the Past is forcing its way through, into the Present!", "An ancient Presence, powerful and pitiless, demanding worship and sacrifice, blood and horror. Out of the Past, out of Time, come to drag Humanity down to its own level again", "I'm getting definite indications of Time shifts. Intrusions from the Past. Some recent, some not. And underneath all that... I'm reading Deep Time, JC. From long before this station even existed. This is bad, JC, seriously bad. I've never seen so many extreme readings in one place before."
That's just in the first three chapters of the book. Later on their speech does creep up to match the modern timeframe of the story, but the overemphasis on "OMG!!!" sits there all along. Each new thing they encounter in the London Underground is always more than anything they've ever seen before. And these are supposed to be seasoned field agents? Green tries to get around that by frequently reinforcing that they are a B-team and only there because there was absolutely no one else. Same with their enemies from a rival organization. That team is also there because there's no one else and they're an effective B-team. However, two Bs do not an A make.
Add in some over the top descriptions, more than one info dump, a deus ex machina and an barely believable team up and the story just falls apart. Which is sad, because I know Green can do better and the idea had so much potential. In the end, it felt like Green was trying to be Lovecraft, rather than paying homage to his style. Bad choice all around.