My toilet has started running on after flushing - water continues to enter the cistern from the top of the fill tower, and flow out through the overflow. It's not a "perfect" failure; if I depress the ball-float and let it bounce back up, sometimes that shuts off the valve and the water stops - but it doesn't shut off every time. I don't seem to be able to stop the water by manually lifting the float (to exert more pressure on the valve).
I'm guessing I need to replace the valve that the float controls, but I can't find a definitive answer or instructions - largely because I'm not 100% clear what that specific valve is called (so this might be a duplicate for the same reason - sorry!)
I've attached a picture of the cistern, with the valve I think is faulty marked, in case this helps diagnosis.