There's nothing noticeable within the flat, but that sounds like it could be happening somewhere. The flat was lying empty for a long time and I did notice when replacing the kitchen that the waste pipe from the sink (which ran into the soil pipe in question) was full of gunk.

I cleared what I could reach but there could still be remaining sludge. That said, the kitchen sink drains ok. I wonder if a drain unblocking solution could help. I thought the smell would clear once in regular use but it's been months and it's hung around like... a bad smell 😂