Dave Baker wrote in message
. ..

Shared drains built post 1st October 1937 are the joint responsibility of
the various homeowners unless the water company has 'adopted' them. It
doesn't matter where in the shared system the blockage occurs. Everyone
connected pays their share.

Addenda - unless your drain joins the private sewer downstream of the
blockage of course in which case it isn't your problem. That will never
apply if there are only two shared drains as in the most usual case of a
semi detached house. However all upstream of a blockage share the cost of
fixing it regardless of under whose land the blockage occurs.
--
Dave Baker