Excellent picture CJB, well done.
Below is my first ever sighting and picture of a Purple Emperor, also at Bookham Common but in 2013. I saw lots of Purple Emperor in 2013 at Bookham but nearly all were high up in the tree canopies.
I didn't capture one on camera at ground level in 2013 despite approximately ten visits to Bookham at peak times following a trail of manure!
After I took this picture I decided I needed a new camera!
I wont be winning any photographic competitions with this picture but at least it shows just how hard it can be to capture these beautiful butterflies on camera.
best wishes,
Philip