Hasn't the 'circle' already been mentioned in one of the Calvine threads? IIRC, someone suggested it might be a stain from a drop of water (etc) on the photo.Using photoshop levels adjustment if you move the middle (midtone) slider to around 0.2 do you also see a circle on/around the "front" of the UFO?
(Three frame .gif: original, midtone adjusted, circle annotation.)
Personally, I don't see the circle in the un-enhanced version; I see it very clearly when it is ringed in red; and in the intermediate version it is difficult to 'unsee' the ringed version.
Added: I did notice something mildly interesting just now. Previously I had my laptop plugged into power, but when I unplugged it to take it to another chair, the screen switched to a lower-brightness mode, and the circle in the intermediate version suddenly looked clearer. Playing around with the brightness level I can either eliminate the circle or enhance it. OK, that's not very interesting: it just shows that the circle isn't an artifact of Photoshop in particular. But while playing around with the image I also happened to see the lower half of the photo in isolation (without the mystery object in view), and it seems to me that there is another definite circle in the image, to the south-west of the object, and above the middle fence-post. More doubtfully, there is another possible circle to the south of the nose of the object. Is it possible that the MoD Press guy once dripped some tea on the photo?
Apologies if the second (and third?) circle have been previously mentioned.