If it’s a big drain, then is it serving several properties? Which would make it the water (sewerage) company’s problem, not yours. There’s been a change in the law. Time to lift a few manhole covers, maybe.
We had water leaking from drain problem. Contacted the local water authority. They had plans of our house on their screen and could see which drains were theres and which were our responsiblity.
If they cant see it on screen then ask them to send an engineer out to check whats what.
Basically if its a pipe that purely serves your house then likely yours to deal with. If other houses stuff goes through it, its theirs.
Push them harder to do some thing.
The guy who came out to ours was actually very helpful, even though it ended up being our issue.