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I would start by pinning it properly so that you can manipulate it under a microscope or strong magnifier. Do you have access to a microscope? I have a page about collecting insects on my blog.

Next, I would try to get to grips with the different families. After a bit of practice you can spot families that you are interested in in the field but there's nothing better than knowing *why* your fly belongs to a particular group - it can stop you trying to key something in the wrong book ;) The standard key would be Unwin,D.M.(1991): "A key to the families of British Diptera" published by the FSC and freely available on their website: http://www.field-..._143_A.pdf

When you have it to family you then have to locate the appropriate key and that should get you to species :) People here can help with advice about family keys.