Continuing on the Pub, the latest AAIB news is that they're puzzled, there was no mechanical fault with the aircraft and it was fueled. The rotors were not turning, however.
It only had one pilot, however, and the question turns to the possibility of human error. One discussion left open was the exact technique of counter-rotation, and that begins to offer a viewpoint: was the pilot demonstrating counter-rotation but misjudged the 3rd phase, Recovery, because he thought the roof was the ground? The phase is, effectively, a flare-out, translating the vertical drop speed into horizontal then killing that in a flare-out much as any fixed-wing aircraft. It's done just a few feet above the ground, less than roof height. Put a roof there and there's no flare, just a crash.