A strange phenomenon photographed over Brecon, Wales

On May 24, 2013, in english countryside (near the city of Brecon, in Wales), a strange aerial phenomenon has been photographed. This singular UFO was perfectly peaceful and invisible. What is it?

Witness statement:

The picture was taken half way up a very steep climb at Storey Arms, Brecon, UK. (Postcode is LD3 8NL and the grid reference is SN 982 203 (OS Explorer OL12 - Brecon Beacons Western Area). Date and time - 24/5/2013 at 12.56.09.

When I took the picture I did not see anything. The frame before and after have no markings, I have taken over 10,000 pictures with this camera (Canon 650d fitted with a Canon 15 to 85 zoom) and have had no similar markings. The day was very windy, visibility was good, a forest was 600 to 700 feet below and the area we were walking in is short rough scrub/grassland, the spots of white in the far hillside are sheep, this might give an idea of scale. The shutter speed was 160th sec at f7.1 at 15.0mm wide angle, 100 ISO, image size was 18mp.

I am familiar with the use of Photoshop but nothing has been altered from the original image apart from contrast, saturation and crop to enable a large zoom image. The first two images have enhancement and the second two are the original image, I can supply the full size original file if requested.