Manholes missing?

Section62

https://forums.moneysavingexpert.com/discussion/comment/82014382#Comment_82014382

It sounds like the extension was constructed prior to October 2011 in which case any drains that were running along the back of the house were private sewers. They could have simply removed the manhole when the extension was built and you've now got blind branches into the pipe.

Drains/sewers at the back of the house could have been public, not private, before the 2011 transfer. The effect of the 2011 transfer was on pipes which were not already public sewers to make them the responsibility of the water company. (There are now post-2011 drains/sewers which are private because the adoption has not taken place.)

Case in point, similar to the 2011 transfer, in the 1930's there was a mass transfer of previously private drainage to the local authority, which are known as 'Section 24' sewers. If the OP's property was constructed before (or served by a sewer already in existence on) 1 October 1937 then it was already public when the extension was constructed.

Also, drains serving one or more properties could have been adopted as public sewers at any time prior to 2011.

In October 2011, any private sewer serving more than one property was transferred into the ownership of the Water Company.

The transfer also included pipes serving only one property to the extent they run in land owned by another party (e.g. a neighbour or highway land)