Brandon Lake is the ‘Invisible Detective’.

Rapidly gaining a reputation across London, he can answer your questions, your problems, your suspicions apparently without ever leaving his dimly-lit office.
Brandon Lake has only one problem of his own: He does not exist.

Arthur (Art) Drake, Meg Wallace, Jonny Levin and Flinch are ‘The Cannoniers’.
Their den is in an alleyway off Cannon Street, and apart from Art they’re all street kids. Art’s dad is a policeman at Scotland Yard, though he hasn’t told his friends that. The others need money to survive –
to take back to their impoverished homes, or in Flinch’s case to keep her alive on the streets. They each have skills that they can use. Together they can find out what’s happening anywhere on their patch. But who’d listen to teenage kids?

So they have invented The Invisible Detective.