Fair point topbunk - I mis-phrased that, we certainly didn't achieve double the miles per gallon of a 747. I should also have made it clearer that I was comparing it with contemporaries, i.e. -100 and -200. (I recall about 2.5 - 3tonnes per hour per engine on a typical Atlantic sector)
So I agree it wasn't double the miles per gallon - although in terms of mpg the Conc was markedly better in the cruise than the 747 classic. Of course, you paid for it in the process of actually getting to M2 as well as dragging around the pattern at 300kts.