That sounds charming. I have visions of a chap in uniform and a peaked cap holding a gate open for any passing rats saying 'Good evening sir, this way please'.
It's just like a tiny farm gate, and then the farmer cat comes along with his ratmouse and does some weird clicking sounds and counts them through "yan, tan, tethera, methera, pimp."
We got a CCTV survey done in the end and it turned out the drain was properly fucked.
There were three problems:
The poorly-fitted rat gate where our run met the main sewer was backing up
The T joint where the downstairs soil pipe met the pipe from upstairs hadn't been sealed properly and was leaking
The clay elbow underground where the downpipe turned to run into the sewer was smashed to pieces
Patio dug up, new rat gate installed, new UPVC pipes installed, wallet significantly lightened, confidence in plumbing restored.