Last master of the Ferris Guild. Dying of the thing the Tube gives you, more honest than usual, and holding the map to a strongroom under the Waterloo Interchange.
The cable-runners' guild, back when the cable-runners still had a guild and a master meant something. Defunct. Its last master sold a dying man's map to the Waterloo strongroom.
A wreck-diver buys a dying guildmaster's map to a Waterloo strongroom that opens four hours a year, at the lowest tide, and seals for another decade after.