I am not a helicopter pilot and welcome criticism.
(1) An unknown roof is not the best place to land. No evidence the aircraft was changing direction to the adjacent roadway.
(2) A great deal of kinetic energy was dissipated rupturing substantial and close floor joists in the compound roof. An internal masonry wall may have been involved. The roof was struck with considerable force.
(3) No evidence the rotors were turning at impact. The engines were not running.
(1) The helicopter was intact descending very fast but not in free fall. By the time the pilot stopped the rolling and rotation ("tumbling") reported by witnesses he lacked height to stabilize autorotation and regain full control.
(2) The immediate forced landing was precipitated by a sudden failure with no warning.
(3) Witnesses heard unusual mechanical noises earlier prior.
(4) There is an AD on the ring frame holding the fenestron to the boom.
(5) Photos of the scene show the fenestron broken off?
Edit:
http://www.caa.co.uk/docs/33/2013120...D20130289E.pdf