An earlier post said the tower would be 500ft high (ie AGL) when completed then speculated about the height of the crane on top. An earlier post said the NOTAM had the obstacle (building + crane + jib) at 770ft AMSL, ie above sea level (so more or less Thames level). By some of the early news coverage the fog was up and down at the time but definitely capable of obscuring the crane and even the top of the tower according to eye-witnesses. The 500ft rule allowed the pilot to come down to land for his diversion to Battersea but horizontal viz would have been poor and on a divert he may not have had that particular NOTAM to hand and he hadn't yet made radio contact with Battersea it seems who may have been able to warn him. Very unfortunate circumstances.