The small Thick-legged Hoverfly is a commonly-found member of the hoverfly family. It is very nippy in flight, so photographing it is very difficult. The thickened rear femur is a distinctive feature - hence its name, clearly visible even in the photograph we have here. They occasionally appear in local population explosions. This hoverfly is sometimes called the ‘Compost Hoverfly’
These flies are important pollinators.