Michael Hebbert, in the A Walking Tour section of The City Of London Walkway Experiment.
Written in 1993, that’s just as true now; however, vast chunks of the system he describes are gone, including the bulk of the Liverpool Street - Leadenhall section.
A random, longwinded observation of my own. Those “secure beneath the watchful eyes” ads that everyone from Boing Boing down took exception to (years after they were actually on the streets)? I was never that bothered by them, either. At least if you’re going to have CCTV, tell people about it.