We had some kind of gizmo put in by Anglia Water, a few years ago, which was supposed to save water when you flushed the toilet. I think it's that bit that's snapped.
Gawd, I hate messing about in the toilet water! I have just compared the upstairs toilet with the downstairs toilet and they don't look the same. The broken one has a piece of loose string.
I will call out the emergency repair team tomorrow. In the meantime, I can flush the toilet by taking the lid off and pulling the metal hook up. What a palaver!
I'm just off to google, toilet cisterns so that I can use the right terminology when I talk to them tomorrow. You all seem to understand me but my friendly local plumber may not.