csd,
Heathrow is very noise conscious so chances are that it would have been a max thrust departure in order to get the shortest take-off roll and a good initial climb.
Would you be so kind as to point out a technical reference showing that this is true or required?
As for reduced thrust, talking about the B747-400, it generally comes with 3 thrust ratings, TO/TO-1/TO-2, the thrust reduction is selected by the operator but 0%/10%/20% are common. There is also another procedure called Assumed Temperature Thrust reduction, this permits a reduction of 25% of the RATING in use, so combining TO-2 with Assumed Temperature, you can actually reduce the engine thrust by approx 45%!
AV8, you will never know what happened on that day
Mutt