We have a toilet cistern with a concentric float valve fill mecanism. Earlier this week, it suddenly started to take much longer to fill after flushing (ca. 15 minutes instead of less than 1 minute previously). Simultaneously, the sound produced by the filling became a much more noticeable hissing sound.
Inspecting it wasn't much help, as I'd not really looked at it before it went wrong. I did notice that the hissing sound is caused by water emerging from the top of the fill pip at pressure. That, combined with normal operation of other water outlets, leads me to believe that the problem is isolated to the cistern.
What might have gone wrong? Is there a simple fix? Or will I have to call a plumber?