Ptkay:
You said: "They were obviously landing below minimums for this location".
ATC said: "Their was fog over the airfield that night, but visibility was sufficient for landing".
You have made a very serious accusation. Were you there at the time? I doubt, it but ATC were.
You have trotted out the half-hourly METARS as your "evidence". The lowest RVR quoted on the METARS is 700 metres. The normal RVR minima for a CAT1 approach is 550 metres but I seem to remember that Katowice was nearer to 700 metres.
In any event, none of this matters unless we know what ACTUAL RVR was passed to the aircraft at the time of the approach before we can even begin to speculate. In this case, ATC state that it was sufficient for a landing so they clearly did not make an approach with the weather below minimuns. The METAR has got absolutely nothing to do with it
If you don't really know what you are talking about then keep quiet. Making such serious allegations could get you into serious trouble.