Looking at all the replies and comments it leads me to believe that there is no easy answer, without a doubt it's teachers who provide the excellence regardless of the type of school they teach in, if a teacher is good it will make a difference.
To go off on a slightly different tack did anyone watch the documentaries where pupils from a private school went to a comprehensive for a week and vice versa, the thing that struck me was the head of the comprehensive was quite on the defensive about state education versus private at the beginning yet by the end of the experiment she could see benefits in the way the private system worked. most markedly, it seems, in the way that private schools encourage self belief and confidence in its pupils.
This argument could go on for a long time methinks!