The one aspect that cannot be aligned with TC's scenario (post 482) is that both a member of the band, and a member of the audience have stated that to the occupants of the bar, it was a two-stage process - initially part of the ceiling fell in, specifically causing someone to joke that "the band had brought the house down", followed a few moments later the full destructive collapse. In the scenario described in the previous post there is only the single, massively destructive, impact. What else might have caused the initial, more minor ceiling damage in the above scenario? Not something falling form the helicopter as the AAIB have said nothing fell off (and anyway, if the aircraft had been in free-fall, nothing could have fallen much faster to get down to ground level several seconds ahead of the helicopter itself), and nor could it have been just downwash if the rotors were not generating significant lift.
All I can think of - and I cannot say it is entirely convincing but it is the best I can do - is a scenario of a more-or-less under control autorotation (as stated elsewhere, strobe effects of either the aircraft's own lights, or perhaps dischage street lighting providing an *illusion* of a stationary rotor), but the pilot realising at the very last moment that he was not above a ground-level car-park, but rather over a roof perhaps 12 feet higher, leading to a rapid and aggressive final lifting of the lever to check the descent, in poor light with limited cues, which actually over-checked the descent and briefly got the aircraft rising before the rotor energy ran out, leaving the aircraft to then fall from perhaps 10 feet through the roof. In that scenario there would be a sharp pulse of downwash that could perhaps have caused the initial limited ceiling damage, and a few seconds between the downwash pulse and the impact.
(Previously I was among those favouring a "progressive roof collapse" as providing the time period between first roof contact and total roof failure, but I have to admit to the difficulty of aligning that scenario with the degree of damage to the nose and skid areas.)
(Edited to include post number for TC's recent input)