St John the Baptist Church sits directly on Cathedral Square, close enough to the cathedral that many visitors barely register it. That is a mistake if you like parish churches, because it gives a more intimate sense of Peterborough's medieval town life.
The church is useful as a scale shift: the cathedral is monumental and monastic, while St John's is civic, central, and woven into the ordinary city. Step inside if it is open, or at least pause outside while crossing the square.