My grandmother's home in York, England, also a rowhouse, had a similar
feature not far from her backdoor. It was capped with a concrete removable
lid but was similar in size to the one illustrated; perhaps a bit
shallower. It served as a cleanout for the drains. Family lore includes a
story of an unfortunate visitor having to fish for dentures in this thing
after a celebratory pub crawl resulted in a toilet hugging session the
night before. Alas the teeth were not recovered and the aid of a local
denturist was required.

This particular feature may have had a similar function but was
subsequently modified after the main drain pipe was disconnected. The
smaller line could be water or gas installed after it no longer functioned
as a cleanout.

 Peter Sattelberger
Archaeologist and Material Culture Researcher
Ontario Service Center
Parks Canada
1800 Walkley Rd,
Ottawa, Ontario
K1A 0M5