Looks like the Enstrom problem is the gearbox. There was a loud bang followed by smaller bangs and grinding, along with severe vibration through the airframe. It was recovered today and will be with our engineer tomorrow morning. I suspect it's thrown a tooth from the main gearwheel which was then trapped between the wheel and pinion, and fragmented. There was no loss of rotor speed or yaw, and symptoms stopped after entering autorotation. It could have been worse, minutes later we would have been in the heli lanes routing through central London.
Otherwise a great day. See you next year.
I'll post again when i get the engineering report.
Stephen