@S Jones Yes, that ties up more with other accounts. I'm probably putting too much emphasis on his earlier statements about 'see[ing] the angle, the speed and the trajectory of the fall'. Angle and trajectory could both be referring to the vertical.
However, I've still got doubts about how much he could have seen of a drop from 4/500ft given the significant blocking by the Cathedral in the direction of the Clutha from that roof. Should also probably note that Govanhill (area of earlier hovering) is to the south on the other side of the Clyde.