And in the case of a spontaneous burst water pipe, the body responsible is likely the pipe manufacturer or the pipe installer. The owner might be liable if they could have reasonably foreseen the event.
But it wasn’t a spontaneous event. Someone literally stuffed up a sewage pipe that was otherwise functioning normally. I think someone who has been stuffing disposable nappies and cat little down their toilet might have foreseen that it could cause a problem. Just as well you don’t have to pass an intelligence test to be a parent (or pet owner).
The OP in this thread had just had a new piece of equipment fitted and it failed, flooding the flat below. That probably isn’t a spontaneous event. It was more than likely a failure of equipment or its installation. They should pay and claim it against their insurance, if they have any.