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Larry,
Join the Dipterists Forum for just £6 as Tony suggests. You will be able to access free copies of all sorts of marvellous keys to families like Tephritidae, Sciomyzidae, Scatophagidae etc.
http://www.dipteristsforum.org.uk/

Once you have joined you get access to a members area where some of these are available. Others are also available from various members who specialise in various families of fly. One caution. You will need a microscope to determine many of these flies and that will certainly cost more than £40!