A hoverfly of genus Xylota, probably Xylota segnis (Diptera: Syrphidae) because you can just see through the wings that the front two-thirds of the abdomen is yellow. Xylota tarda is similar to X. segnis but slightly smaller and is not as common. Most other British Xylota have darker abdomens.
Xylota segnis are not a great fliers. You are more likely to see them scrambling over leaves. I have never seen one hover.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-06-20.