The quote above is one of the inscriptions in the Old Bailey the name given to the criminal courts in London and named for the street on which it stands. It is close to Newgate prison.
Old Bailey also makes a cameo in a children’s rhyme, “Oranges and Lemons.” Like many old rhymes there seem to be many variations. I have included one below. When will you pay me feels appropriate for Old Bailey in Neverwhere, although I have also heard the perhaps apocryphal story that it refers to the bells of Old Bailey ringing and being used as a signal for hangings at Newgate prison which lends a darker tone to the concept of “being paid”. Old Bailey is linked closely both to death and to the obscure bartering system that governs London below."Oranges and Lemons" say the bells of St. Clements.
"You owe me five farthings" say the bells of St. Martins.
"When will you pay me" say the bells of Old Bailey.
"When I grow rich" say the bells of Shoreditch.
"When will that be" say the bells of Stepney.
"I do not know" says the great Bell of Bow.
Old Bailey is also the court in A Tale of Two Cities. Old Bailey in Neverwhere is certainly a Dickensian creation. Unlike the other denizens of London Below he is a self-proclaimed “roof-man” and a costermonger branching out in his business from birds to information. Like a lot of characters in Dickens he is likeable but one suspects he is often be on the wrong side of the law if the law is anything he stops to consider. He occupies the other neglected spaces of London - the rooftops.
In looking for images of the Old Bailey I stumbled across this website, which is part of an effort to put cases from the Old Bailey between 1647-1913 on line. It is not particularly relevant to Neverwhere but it was so interesting I felt I should add the link.