I'm presently pulling together Dolichopodidae data for Leicestershire & Rutland (VC55) in order to produce a status report for the family (to 2020) as part of a series of reports produced by the Leicestershire & Rutland Entomological Society.
I have worked through available online databases as well as local records and also pored over publications. I have a simple question - are the data on the NBN Atlas up-to-date and, if not, is it possible to access a database where the information can be retrieved?
Have you tried Martin Drake, Ray? Have a look at the Recording Scheme page on this site via the Resources menu (or use this page address: https://dipterists.org.uk/empid-allies-scheme/home). Notice though that in the comments he says an atlas will have to await publication of his monograph on the Family so I don't know how easy it would be to extract a list of records for a particular County. Good luck.