shiny body, shimmers green in light
No interactions present.
There are several different green flies, from different genera, and you would need a far sharper photo to get to a species. Have a look at the links below...
My Flickr photos...
Thanks for those links Ian, and thanks to Tim Worfolk for adding them to his blog, they explain the situation very well.
Entomologist and biological recorder
Lat/Lng: 51.8, -2.5
OS grid ref: SO6713