Between the cracks in society sit The People Inbetween - the homeless, the dispossessed and the destitute. This lack of visibility, combined with the residual magic of London has manifested a ... shading of reality, especially deep within the tunnels of the tube. The result is a hidden community, down tunnels that twist widdershins from reality, of the People Inbetween.
These underground shanty towns are home to some of the most observant people in the city. Unseen, the Inbetweeners can loiter and watch without any threat of being hassled or moved on. As such, they are excellent at seeing and hearing things that other people miss.
Of course, this information doesn't come cheap - there is always a price for working with the People Inbetween.
(Clearly, clearly inspired by the themes of Neverwhere by Neil Gaimen)