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Well, my dear fellow bibionists, I am happy that we all agree on B. johannis!
Theo: you are much too kind, I did not "rediscover an important feature". Verbeke describes that row of dots for johannis and says that in varipes and lanigerus the black dots are more spaced or less conspicuous and in any case do not form a continuous longitudinal row. - The hint to Haarto (2012) was very valuable. I found the paper online, and it looks very comprehensive, thanks! Interesting to know that this dark row of dots is formed by sensillae.
Libor: I'll send you an e-mail.
Last night the temperature was very low, there was white frost on the roofs in the morning. I wonder if the Bibios endured it. My search for B. varipes continues! - BTW, according to Haarto B. johannis flies in Finland from mid May to mid June. Obviously the flies leave NL after May 5th to arrive in Finland in time :D
Best wishes, Sundew