Unwin, D.M., 1981
A Key to the Families of British Diptera
Well illustrated user-friendly key enabling identification to family level, that avoids jargon so is accessible to the non-specialist. However, it has been suggested that the acalypterates are over-simplified so that the many exceptions are not allowed for, so you may still need to use Colyer and Hammond or Oldroyd's Royal Ent. Soc Handbook.
The work is downloadable from the FSC website.
There are companion volumes on Bug and Beetle families by the same author.