I had forgotten about the Swindon-Pewsey bus which I'm sure I've seen going through Avebury. They may have changed the route as I can't see Avebury on the time-table but it must go quite close (stops at Oare before it gets to Pewsey).
http://www.wdbus.co.uk/site/uploads/publications/157.pdf

I was thinking about the people who stay over and camp on the Saturday around the Summer solstice - a lot of them come on public transport and would effectively be stranded if they can't get back to a train station the following day.