Pond fly

There have been loads of these flies flying around our pond these last few days in the hot weather, settling momentarily then moving on. This one is on a rose petal floating on the surface. They don't seem to me to be feeding, laying eggs, resting, or anything. The nearest my insect book seems to come to them is Lonchoptera lutea, but it doesn't seem quite right. They're about 10-12 mm long.