This bug was sitting on a Yellow Flag Iris in a marshy area near some coniferous woods.
No interactions present.
I'm sure you're right. I looked up Google Images with the latin name and this does look like a Green-legged Sawfly. Many thanks for the ID.
I'm not sure about the id. but the spelling is Tenthredo mesomelas, which is why other images and info aren't showing...
Lat/Lng: 54.4, -7.1
OS grid ref: NV7214