wrote:

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.

That is my understanding too, but I only found out AFTER the blockage.
Still, the satisfaction when all that ****e suddenly went WHOOSH was
definitely worth it. That's the second drain I have unblocked. So the score
so far is me 2, Dyno-Rod 0.