Despite the common name, this is really a sawfly (Symphyta). The pine fence-posts which we were erecting on the site seemed to be the attraction to this one, which is a female. Female Horntails have a very long ovipositor, visible in the picture, which is used to lay eggs in timber, and they do, apparently, prefer pine.

The one in the picture thoroughly investigated other posts on a van, but finally settled on one of the newly-erected posts, where it was soon joined by another of these imposing insects. Despite their large size - the bodies must have been 4cm or more in length - they were quite peacable, and seemed oblivious to the people and cameras; intent on finding the right spot to lay their eggs.