St Mark's fly

Strange weather here today. Quite chilly by the river but every now and then  there would be a 10 minute interval when it would actually feel summery. This seems to have brought out large swarms of black flies with very long back legs which trail beneath them when they fly. 
A quick bit of research tells me they are Hawthorn flies, or St Mark's flies. Their trailing legs and habit of landing on anything, including people, makes them appear quite alarming at first sight. However they don't sting or bite and are very useful pollinators so don't swat them please! They also go by the name of St Mark's fly as they always seem to appear on 25th April, St Mark's day. 
A more detailed study of St Mark's fly at rest in the extra.