Hornet-mimick hoverfly Volucella inanis (Diptera: Syrphidae). Compare with the real hornet here. Volucella zonaria is similar to V. inanis but is noticably larger and the first light band on the abdomen is reddish-brown rather than yellow.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-09-04.
The above fly is a female (its eyes do not meet on the top of the head), the following one is a male (its eyes do meet):
Photo taken in The Wilderness, Reading University grounds, Reading, UK, on 2010-07-20.