In reply to captain paranoia:
> Pretty sure there's a chunk of solid PVC pipe at the bottom, possibly painted. And that it's not a spiral reinforced ducting; if it is, the reinforcement is spectacularly bad...
You’ve convinced me I was wrong.
> It's a right old mess, that's for sure.
Yes; if you’re right it does look like an unsealed inspection hatch - with a sponge in it. The pipe down below it (or subsequent drains) must be at least partially blocked for things to spill out of the open hatch.
> Not sure which country it is; internal drains still not common in the UK, I think.
The building my labs and offices are in has internal gutters for rainwater, as did the previous building I was in. That also had an internal downpipe across the ceiling - for an upstairs toilet - in one of the offices. You could hear jobbies sloughing across the ceiling. Award winning building from a big name Architect who only gave a toss about the external look and clever construction style.
I’ve never seen a private house with internal guttering in the U.K.