Found this on the MPs this afternoon. (Maris Pipers).
This looks very similar to the Drone Fly (see Dronefly post), but with one very key difference.
Look at the markings (pale horizontal bands) on its thorax - these are very distinctive, and something that the Dronefly does NOT have on its thorax.
This, therefore, is a hoverfly by the name of Myathropa florae - although it really should be called the Yellow-Dusted Hoverfly in my book - its fat body hair seems to be covered in pollen.
Relatively common in gardens, it breeds like the Dronefly also - and the 'rats-tails' found in stagnant water in tree-holes could well be the larvae of this species of hoverfly...