I think we know this one. The St Mark’s Fly (Bibio marci) is common during the spring around woodland edges. We normally see it drifting along with its legs dangling underneath but the Wildlife Trusts website says that they often land on any objects in their way and are rather sluggish at rest.
They are named after St. Marks day, 25th April, when they appear with amazing regularity. This one was 2 days early though, which may explain why there were no more around. It was just meandering along aimlessly on the forest floor, presumably wondering where all its mates were. No, I don’t know how they deal with leap years.
The larvae live in the soil feeding on roots, grasses and rotting vegetation, and are often found around compost heaps. The adults feed on nectar and are considered as important pollinators for fruit trees and other plants. Males and females look very different: the male has very large eyes, while the female has a small head with tiny eyes. This then, is presumably a male.