Root-maggot flies?

Root-maggot flies?

Observation - Root-maggot flies? - UK and Ireland. Description: Each year these flies sit around in groups on low leaves (eg bramble leaves, flat and close to the ground), in a variety of places in Abney Park. I can't pin down what they are. If you can help that would be great.

Each year these flies sit around in groups on low leaves (eg bramble leaves, flat and close to the ground), in a variety of places in Abney Park. I can't pin down what they are. If you can help that would be great.