"In my mind I was speculating that we might have read about a catastrophic main gearbox failure"
I was fully expecting the same.
The AAIB report specifically states the main rotor and tail rotor were not rotating at impact. Witness reports indicate "misfiring" sounds.....presumably engine surges. So presumably the engines were running very shortly before the accident.
How do you go from engines running to rotor stopped in such a short time frame.......you'd need both engines to stop more or less at the same time (pretty unlikely) and for the pilot to grossly mishandle the collective (even more unlikely). It beats me.
I really hope we don't end up with a final report where the cause "could not positively be determined".