Terrible tragedy and it's bound to have implications on many levels.
HeliStudent - Yes - Cranes are NOTAMd and displayed on SkyDemon etc., but if the Crane is NOTAMd or not seems not to be the point if there's a substantial building right next to it.
When you look at the display of Skydemon (and other GPS) you wouldn't be able to use it to stay centrally over the Thames at 90Kt. as the resolution is too low.
I'm sure there's loads of speculation about the cause (it may not be related to the visibility or location of the crane) but IF the flight was being conducted SVFR then conditions should have been appropriate to see buildings - and I'm not speculating if it was or wasnt, unlike Sky and everyone else !