This ungainly looking fly with a long, straight proboscis is Empis tessellata, seen here feeding on hogweed nectar. That isn't its only reason for lurking on hogweed umbels though - it's a predator of other small flies that are attracted to the flowers.
Individual hogweed flowers secrete nectar from the surface of the ovary, from which a pair of styles with stigmas at their tips arise. The nectar is a very convenient energy source for small insects. By providing small amounts of nectar on numerous flowers, the plant ensures that the pollinator must wander all over the surface of the umbel to collect it, accidentally pollinating stigmas of many flowers in the process.
Empidid flies are sometimes known as dagger flies, on account of those long piercing mouthparts. This is the largest British species.
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