On Sun, 23 Jun 2019 09:11:38 +0100, "Brian Gaff"
wrote:

Were perhaps the toilets or toilet retrofitted, IE the breathing at the low
level is very much older than the new one? Are we sure they are on the same
sewer? If it is, personally, I'd put a bung in it, but there you go!
Brian
+1
I had similar and just cut it off below ground level, bunged it up and
laid the patio over it. There's a nearby external inspection cover for
the sewer, and a drain gulley, so if the sewer ever gets blocked any
overflow will come out there rather than find it's way into the
house.