Hoverfly Volucella zonaria (Diptera: Syrphidae), a male from the side. And from above:
Note that the yellow bands are much darker towards the front of the abdomen.
A few seconds later the following similar hoverfly landed on a neighboring thistle flower head:
This a a male Volucella inanis (also Diptera: Syrphidae). Here the yellow bands on the abdomen are all evenly yellow.
Photos taken in the field below Chazey Wood, near Caversham, UK, on 2011-07-22.