So if they were stable, with approach power set one can assume that the autothrottle wasn't asking for that much power when the engines failed to respond, surely they must have had considerably less than 'approach power' on for this to happen...unless of course the engines actually lost power (which the AAIB didn't mention). What sort of N1 would a 777 have on for an approach like that?
Did you consider that maybe the wind conditions on final are not always constant?