[Note added 2011-11-20: I now think that this is probably an Eristalis sp.]
The name of this species is occasionally spelt M. florae, that is with the 'a' before the 'e'. I am not sure whether this is a proper synonym or just a mistake (this form is used in Complete British Insects by Michael Chinery, Collins, 2005).
I am also not entirely sure about the identification; the legs seem a bit too dark and the 'knee'-joints lack the light colouring that is shown in most photos of M. florea. The following photo better shows the wing venation and the patterns on the upper surface of the abdomen and thorax.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-09-24.