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I have been for a walk to the gallery room, and there I found two photos of Platycephala planifrons that look remarkably like this, one of them at least was determined by Paul Beuk.:D

I also found a photo of Trigonometopus frontalis, very similar, but with bristles and maybe some spots on the wings and much less obviously polkadot elsewhere, so I am going with the Chloropid.