No it is not a robber fly.
It is a wasp in the Ichneumonidae family.
But I can't find a good match for it. There are quite a few that have a white band on their antennae, but none that are otherwise all black, as far as I have found.
There's a good sample here - http://www.naturespot.org.uk/taxonomy/term/19403.
And this one does look very much like yours, but I suspect its caption (Ichneumon suspiciosus) may be wrong. Most other photos using that name show an insect with white parts on their legs or other different colourations (yes, some of that can be due to sexual dimorphism).
There's another similar one here, but it is not named and it is in South America.
We could do with some help from others on this forum, please...
Meanwhile, Avondale, have a read of this