A Regal Fly

This fly was on a window. I have never seen one like it before. It might be some sort of hoverfly. It was bigger than a blowfly. The black panels either side of the blue shield were like silk velvet.

I had to use a flash to get the detail against the bright outdoor light.

Edit: Steve on Naturewatch has identified it as  Helophilus cingulatus.  As I suspected it is a large bodied hoverfly.