I have added a bit more drama to the star trails and increased the highlights on the horizon to give more of a silhouette from the stones. I have also removed the bushes on the far right as I found these slightly distracting and thought they imbalanced the shot slightly. I will delete the previous edit in a few days.
This is a composite image. The background star trails were taken on a recent shoot in Saltburn By The Sea and the foreground was shot during the day on a trip to Stonehenge back in May 2013. This is the kind of image I imagined getting on that trip but due to the site being heavily guarded at night I couldn't gain access so I had to improvise on the picture I wanted to create.