"I'd salute, but you've got my arm," she tells him, looking (totally not) woeful. "This way's a shortcut."

The way she leads him is a tiny, cramped alley just off the one they originally came down, filthy but deserted, and it opens out within shouting distance of the canal. Fifty feet above them, a chain of carriages rattles through the gap between buildings, louder than a tram but quieter than a train.

"That's the Overground," Nix explains, when it's gone and she can speak without shouting. "They built it nearly twenty-five years ago, to replace the Underground. The old tunnels are where I live, now."