Which door was opened first and by whom? And was this the trigger for all the events that followed?
I don't know which door was opened first. It appears from William's post that the rear cabin may have heard an evacuation call so it is possible that the cabin crew also heard this over the interphone or were alerted by some other means not evident to passengers who are unaware of BAs emergency procedures.
I was in the rear cabin, seated at 45E, and there was deifnitely an evacuation ordered and it sounded to me to be the captain although it could have been another member of the crew. There was an alarm, a "nnnnnn-nnnnnnn-nnnnnnnn" sound.
This PA was not heard by myself or any of my colleagues. About the the only part of the cabin that we didn't have someone sitting in was the rear cabin so it is possible that the whole PA system was affected forward of where the sparking/smoke was. On thinking back I may have heard the alarm, but I assumed this to be a smoke alarm.
As a trained professional what was your mind state at the point of seeing smoke?
My immediate reaction was to get a fire extinguisher, but since I was not familiar with the locations or the operation of the equipment on BA 747s, I thought I would try to get one of the doors opened and help people out instead. I was never overly panicked as I was happy that we were on the ground. If we had been in the air, it could have been much more serious, especially if we were mid-Atlantic.
I think that overall it was a good experience to be involved in a successful evacuation with only minor injuries. Later on at the hotel, I spoke to the only passenger who was hospitalised and he was going to be fine.
What is the device that wasn't working, emitted blue sparks, and had caused people to be moved from their seats earlier?
IFE stands for in-flight entertainment, in other words the TV screen.