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Many thanks Piet for this quick definitive identification. This is a new family to me - and this fly is rarely recorded in the UK, but no doubt much overlooked as it was very well camouflaged on the old rotting tree I found it on. I will inform the nature reserve where I found it ( a famous ancient woodland in Gloucestershire) and the county recorder. The behaviour was fascinating to watch - and I have now found a nice vide clip on wikipedia: http://en.wikipedia.org/wiki/Clusiidae although what I saw was more advanced with pulling and head pushing contests. I may go back to try and get more shots / video myself. Also good to see other observers on this site thought it might be Pallopteridae, though I had a feeling the body shape was wrong.