Confidence: It's likely to be this, but I can't be certain.
Notes: I'd addressed myself to the task of deciding which Rhogaster, and had noted the green pterostigma and black marking on the mesopleuron. Then I noticed that vein 2R, which runs across the tip of the wing towards the pterostigma appears to be missing on both wings. If so, it is going to belong to the subfamily Nematinae. As a quick check, R.viridis is quite big - 10-13 mm bodylength. I THINK all the black and green Nematinae are smaller than this. I quite agree, apart from this problemette it looks every bit a Rhogogaster!