This is an old theory, stemming from the discovery that the solstices are marked. I'm not really convinced. I do think, absolutely, that the henge marked the solstices.
I think Darvill is squeezing his data to fit his hypothesis. I think it's notable that he doesn't cite any of the recent research about Stonehenge as part of a ritual landscape, the data about how it was used, or it's connection to earlier ritual henges.
BUT I think they didn't care about the rest. It's not important for their agricultural or ritual uses. Moreover, we have heaps (literally) of recent data about the construction and ritual use of Stonehenge, and it's incorporation of earlier henges from the nearby river Avon, and from Wales, and two annual progressions and feasts on a regular, trafficked route, with people living on site with animals for the duration of the ritual feast, at which hundreds of animals, mostly pigs, were consumed.
If anyone wants references, I'll post them. I'm updating my Stonehenge FAQ, so have been deeply immersed in the recent research.