---

Hi Joris.
It is H. velutina, though 19.7 is a little bit late for this species. H. velutina is one of the few common Hydrotaea and it is really unpleasant that it is placed in the end of the key after excluding all the rest species. I'm working now on the new key for Palaearctic Hydrotaea, but velutina is really Hydrotaea nothing-special!
In your case we can exclude H. pilipes (uncommon species) for following reasons:
upper parafacials dusted (glossy in pilipes)
t2 without ad (present in H. pilipes)