Originally Posted by rustle
IMC rated pilot cleared on the approach passes through height 1000' descending, ATC may pass further viz/RVR and if it drops below 1800m the pilot must go around.
Not so. The pilot must make an assessment of flight visibility below cloud. It would not be unusual for the met vis as reported by the tower to be very different from the flight visibility on approach to a particular runway. The RVR is significant only in relation to the absolute minimum RVR for the approach.