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.
Recent keys to Psilidae by Withers & Claude and an online key by Paul Beuk may prove to be particularly valuable.
Other published keys are recommended in the Summary to each taxon. The majority of the papers cited are Open Access.
The FSC Identikit is one of the Biodiversity Projects initiated by the UK's Field Studies Council.
Multi-access online keys to European Micropezidae & Psilidae:
This is the first ever FSC Identikit implementation on a Scratchpad site. They are currently being developed further and it is hoped that it will eventually be possible to run them within this site using Scratchpad resources. If you wish to collaborate in their development then do contact me
Developers may find the FSC Biodiversity Forum of interest.