I have just reported data as described to me by my friend. His estimates probably do include large errors, but the event was startling to him and the 4 colleagues with whom he was travelling. The cabin crew were alarmed by the event, saying that they had never experienced anything like it, and the FO looked "white" as the passengers disembarked.
My friend is going to let me know the flight number so that hopefully we can ascertain the aircraft type. He travels frequently by air, but is not an enthusiast so does not recall the type. He is a very capable scientist by profession and not prone to exaggeration; he has also flown with me in a fast jet aircraft so has experienced aerobatic manoeuvres and extreme attitudes. One of the colleagues travelling with him has been rear crew on the Nimrod fleet and had never experienced such an event which, having frequently experienced 60 degree banked turns at low level, gives an indication of the magnitude of this incident.
This is a genuine post. If anyone wishes to discuss this further by private e-mail, I will obviously respect their confidence. Finally, Crotalo, no offence taken; I understand your scepticism.