Phil Anthropist wrote:
The shared drains issue is quite common with older properties. The drain
from my 1900 semi runs from the back of my house to the back of the
adjoining semi, from where it runs to the front of my neighbour's house. So,
apart from 2 to 3 metres under my neighbours front drive, then to the
pavement and road, the drains are all under the two properties. I have had
one blockage so far, but a garden hosepipe and toilet plunger on the drain
opening fixed that. It took several hours but Dyno-Rod at around £200 just
wasn't an option.

If it is a shared drain for a house built before 1937, it is a public
sewer from the point where the waste from more than one house joins the
pipe, and it is the sewer company's responsibility to maintain it.
They also have to sort out blockages etc within 4 hours or so, I
believe. No doubt they are grateful to you for doing their smelly
unpleasant work for them, though.