Witness A
AAIB said "Radar contact with the helicopter was lost at 2222 hrs.
Around this time, the helicopter was seen and heard
by a witness who described hearing a noise like a loud
“misfiring car”, followed by silence. He then saw the
helicopter descend rapidly."
This looks key. AAIB have had a week to interview numerous witnesses. They chose to quote only one in the entire report, perhaps because of the high confidence in that witness or because he was representative of the collective witness statements.
Sound: Notice the abrupt transition from "misfiring car" to "silence"
Visual: "He then saw the helicopter descend rapidly". He heard it before he saw it, in fact he may not even have seen it before the descent. How clearly could he see it in the dark at all, lit by just normal navigation lights? No one has mentioned a searchlight at anytime.
So what event creates a sound like a misfiring car, then silence then instant vertical descent with no auto-rotation and rotors arrested so quickly that there is no residual rotation just a few seconds later on impact? The answer lies therein.