Shower drain finally unblocked. Looks like this had been slowly happening for a long time (like YEARS). We tried everything from caustic soda, bleach, plughole unblockers, even this thing

None of them worked until we got someone in to look and they narrowed it down to the joining pipe leading to the sewer pipe. When they removed it (and drained the backed up water), the joining pipe had picked up a huge amount of black stuff, possibly rust, not sure. They cleared that, put the pipe back on, and the water started flowing through.

After nearly three days of not being able to shower properly, I savoured being able to clean myself properly :)