As the wheelie bin was quite honky, I thought I'd give it a clean.
Screwfix degreaser in a bucket, top up with water, slosh it in and scrub away with an old yard brush.
Upon lifting the cover from the pit on my drive (down which I was going to tip the stinky water), found that the pit wasn't draining, and there was a foot or more of bog paper and turds gently circling in the gloom......
Off to
Screwfix to buy some drain rods (lost my old ones) and shoved 9m down there, with no joy.
Went back, and bought 4m extra rods.
Eventually, after a bit of back-and-forth (trying to get a water hammer action going), pulled the lot out and was very relieved to see a very strong draining going on.
Slung the hosepipe down there, and ran 20 mins' worth of water down it, to make sure it was running properly clear.
Don't know what caused it though, as a bit of bog roll is the worst thing that goes down there.......