Angels one five.
What had happened was that mist from small stream flowing through the airfield was producing a shallow bank of mist drifting through the touchdown area. The rest of the runway was clear, yet 50m was being reported.
Just interested to know where the RVR detectors are located, and why the controller just couldn't say that some mist was drifting across touchdown area, instead of saying 50m RVR when the runway could be seen from 10 miles out @ 3000'.
By the way the mist was 6 feet above runway about 2 foot deep.