Old Men, sitting by a fire with cups in hand, telling tales of Old Days, before the changes. London is a mysterious town, full of strange places, and Northern England even more so. Machen encapsulates his entire raison d'être as a writer: sometimes worlds may overlap. The universe is a fluid place, and may at times allow visions of before and of elsewhere to appear now, and here.
Gentrification is a topic in this short story; the pulling down of the archaic, to be replaced with the prosaic. As I am also an Old Man (or soon to be), I found much that resonated in the sentiments expressed. Ah, the San Francisco I first moved to, in 1994. Strange, full of mysterious nooks and crannies, odd shops, odder people; now a shiny tech paradise full of tall bright buildings and tall uninteresting people. Alas!
But back to the story: "perichoresis" is most usually defined as a way of describing the relationship between the Holy Trinity: a blending, an overlap of sorts, but most of all a rotation. It may be used in other ways as well... to describe an interpenetration between the world we know and a world - or a person - that has passed, or that exists elsewhere, now only to be seen in fleeting visions.