Flies are tricky things but we’re pretty sure this is a noon fly (Mesembrina meridiana) sunbathing on the picnic table in early May. It likes well wooded and well vegetated area. NatureSpot confirms it’s love of a sunny spot and it is this habit of sunbathing which gives it its nickname of the noon day fly.
Wikipedia adds that eggs are laid in cow dung (though apparently other mammals will do), and that the larvae are carnivorous and feed on other fly larvae within the dung. According to that niche best seller ‘Insects of the British Cow Dung Community’, the female lays up to five eggs in a lifetime, each one in a different pat, at two-day intervals. Presumably by only laying one egg in each pat it ensures that they do not eat each other.