The European Families of Diptera
A recent favourite is Oosterbroek, P. (2006). The European Families of the Diptera: Identification, diagnosis, biology. DOI: 10.1163/9789004278066 (link)
In 2008 Stuart Ball produced British Families of Diptera to support a workshop. These are only available to members of Dipterists Forum.
If you have visited this site because you think you are generally in the right area then the images may be of the most help in narrowing down the species.
Go to the Media Gallery and select Photographs, then use the European Species list to explore further. There should be enough clues at each taxon to help narrow down your choices. If you remain uncertain then try iSpot, Diptera.info or Dipterists Forum.
Please check the known distribution from the maps, a species known only from North Eastern Russia is unlikely to turn up in a Surrey garden. All known British species have a UK distribution map amongst their images.