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I found this tachinid on yarrow in july in north london. The dusted tergites suggests L. anthracinum which is apparently common in south europe so it's most likely, but how can I be sure it isn't one of the other 3 possible species in the key? eg crassum, tetraptera or turonicum?

It seems to have 2-3 rows of hairs on the parafacial.

Can the frons width be used to determine the species?

I cannot see a difference between the dusting on the hairy part of the cheek compared to the hairy part. There's also something about the tergites and some diagrams which I can't quite figure out unless they are dorsal views. Are close-ups of the genitalia likely to help - they're hooked out and ready - or is it just that view of the last tergites that's important?

Jeremy