Cloud with some sun.
Ectemnius Wasps are predators on hoverflies: They capture them and take them back to their solitary tunnels for food supplies. I rather like to think that they are Hoverfly mimics. The top image shows the large head in relation to the thorax: Hoverflies have large eyes.
The abdominal colouring has more black than yellow - another feature of many hoverflies. Notice those two-tone antennae.
The next two shots show the difference in the face when the antennae are held down (left) and up (right). I suspect the antennae are held up when approaching a hoverfly - the face is much more hoverfly-like when that takes place. The antennae will also appear to be shorter in that position, and hoverflies have short antennae.
Since the hoverflies adopt wasp-like colouration as a means of self-protection (Batesian mimicry, where a harmless creature pretends to be a more dangerous one), I like to think that this wasp is pretending to be a hoverfly that is already pretending to be a wasp.