Steve Jones in the Guardian is satirical about the idea that Stonehenge was a complete circle.
It's actually not such a silly thing as you might imagine. Due to the lack of a complete set of stones to make a circle, some sensible archaeologists have wondered whether Stonehenge ever was complete, or whether they simply ran out of stones of the right size. The discovery of the site of the stone holes does add a bit more to our understanding of this site.
And, if you're going to make fun of archaeologists, it's worth noting this sentence:
"The geometric feat of mapping out a circle, presumably using pegs and cord, was no mean achievement for a pre-literate people."
To draw a circle, you put a stake in the middle of where you want it, you tie a rope or cord of the desired radius to it, and you go round in a circle at the end of the cord, marking your progress with small stones, pegs, powdered chalk or whatever your heart desires. It would have been dead easy for a pre-literate group of people - literary being a measure of a certain kind of progress rather than intelligence. If you want proof of this, read below the line on the Comment is Free page.
In fact, many of the "circles" aren't actually very good (though Stonehenge is), suggesting the Beaker People responsible just stuck them roughly in place, then jiggled them about until it looked about right. The ditch at Avebury, also, is not a true circle
In passing, the definition of a "henge" is a monument having a ditch inside a bank rather than outside - its poor defensive possibilities meaning that it's unlikely to be a fortification, and therefore it's ceremonial (i.e. we don't know what it's for). Stonehenge, having its bank inside the ditch, therefore is not a henge - even though it gave half it's name to them. It's a rock 'n' roll world, is archaeology.