I have had a partially blocked kitchen drain for quite a while and, at last, cleared the pallet piled up with wood that was over the manhole cover closest to the blocked drain (kitchen outflow/gutter downpipe). I had thought that the blockage was up near the kitchen outside U-bend and was intending to try to water jet up from the other side of the blockage.
The manhole cover was corroded to the frame and took some time to free up. When I was able to remove it I was somewhat surprised to find a sheet of plastic 'board' in the cistern and it must have been there since ??????? When I say "board" it is more akin to 'floor' in the bottom of a collapsible dive/sports bag.
Anyway, it was pretty gross because it was slowing the inlet from the kitchen/gutter pipe into the cistern and I assume that it's braking effect on the waste flow was sufficient to aid the build up of a fairly long fat-plug. Also, toilet inlet is in that cistern so I didn't stop to measure it as I removed it (whilst wishing I had longer arms and a clean glove to pinch my nose/it's a wonder I wasn't sick) - it was originally oblong (maybe 1' * 2') but had curled into a tube shape.
I wondered if it was supposed to be some sort of inner cover to prevent unpleasant odours wafting up but it didn't seem to be the right size.
I suppose it might have been put in there to stop pug getting into the drains when building works were going on and forgotten about?
Anyway, I was curious to know if it was a proper part of the cistern/manhole cover setup or a mystery?