Prepared for a Dipterists Forum workshop in 2004, keys to Nerioidea & Diopsoidea seem to have proved useful. They have been updated a little since then, notably in 2008 when a number of attendees at a similar workshop (different topic) took specimens of Imantimyia/Loxocera from which was developed the list of features described on this site. The keys are sufficient for UK species though they are poor for Chamaepsila, a difficult group.
Other published keys are recommended in the Summary to each taxon. The majority of the papers cited are Open Access.
Multi-access online keys to European Micropezidae & Psilidae are currently being developed and it is hoped that they will eventually be placed on this site. If you wish to collaborate in their development then do contact me