Ptkay:
OK. I will play along with you just a little bit longer.
The cloud base plays no part on the decision as to whether to start an approach or not. The only factor that matters in the decision process is the RVR.
For a CAT1 approach, the DH (Decision Height) is set as the minima and this would normally be around 200 feet above threshold on the servo altimeter. Below that height, thou shalt not go unless the required visual clues are there and sufficient to make a safe landing.
Now this height normally equates to an RVR (not visibility) of 550 - 600 metres.
Therefore, the decision as to whether to proceed or not is not predicated on the cloud base but the RVR. As far as the decision making process is concerned, the RVR is the only factor but the crew must not go below the DH unless sufficient visual clues are evident at DH.
This crew obviously found the runway otherwise they would not have been able to land and taxi to the gate.
What remains in doubt is what they did between making the decision to land and actually striking the runway (making use of all available cover).