Well I was watching it and the water is not flowing through the polythene pipe, definitely coming out of the side where I've indicated with the arrow in my 2nd pic
Thanks, but can't see anything immediately apparent. I've uploaded a pic if that helps. The white part at the top right is where the water gushes out and hits the side of the inner cistern wall
I have a fairly modern push button cistern. When I press to flush, and the float arm lowers; the water splashing against the side of the inner cistern wall- at the top- has become very powerful (presumably coming from where the ball valve has opened?) and proceeds to splash over the lid and...