What they are investigating is the circle of bluestones: rough cut stones originating from Preseli in South Wales, about 150 miles away. These form part of what is believed to be the first stone- built phase of the henge, which originated about 500 years earlier. Up to, and including, this time, it was similar to many henges found all over Britain, the extraneous origin of the bluestones being the only unusual feature.
The post- and- lintel stones for which Stonehenge is (as far as I know) unique in Britain were added a few hundred years later, and the bluestones were believed to have been repositioned.
It's obvious that over a thousand or more years of use, beliefs about the henge went through several transformations, as they have continued to do since. I'm not dreadfully clear about what kind of evidence would prove an 'oroginal' intention of a particular function.