I have to say that this insect is one of the most unusual I have ever come across; when I first encountered it sat on bracken I could not even work out what sort of insect it was! I managed to get a couple of slightly blurred photographs before it flew off and armed with those photos I eventually worked out what it was, the golden horsefly (Atylotus fulvus). It has the most amazing large green eyes.
One of the reason it took a while to identify was that although I thought it might be this species my book said it is very rare and found mainly in wet heathland mire in the New Forest and on the heaths of Surrey. This one, being on Wareham Common, did not fit that at all but on enquiry to people that know these things it seems this may be the third record for Dorset and it does have a wider distribution than just Hampshire and Surrey. It is far more common in mainland Europe.
As a horse fly it is, as with its cousins, a pest to livestock, especially horses.