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Thanks Tony,

My first thought was Rhinophoridae too, until I came across Periscepsia carbonaria :

https://diptera.info/forum/viewthread.php?forum_id=5&thread_id=66912

Problem is, that Periscepsia in Holland seems to be restricted to the coastal area, whereas the place where I found it is as far away from the coast you can get in the Netherlands, and moreover it isn't really sandy.

Ben