The evidence of the witnesses quoted by Caz indicates patchy fog, not a hillside or crash site obscured by it. How else could one of those witnesses listed state he saw 500 yards? And another, not listed, state "a few hundred yards".
Actually, Caz's post is very interesting. By going back further in time than MoD would wish, he talks of scenarios that would have "precluded flight".
Something far more fundamental should have precluded flight. An aircraft with no clearance to fly. The question then becomes - Why was this fact withheld from aircrew and inquiries?
Now it is up to Lord Philip to assess this evidence.