A tiny fly with his own festive decorations

This little fly is about half a centimetre long and was sitting on a flax leaf. . He is all dressed up for Christmas with his shiny green bauble and rainbow tinted wings.

I hunted the insect book but couldn't find what he is.

There are a lot of his brothers and sisters dying on my windowsills. In the morning when I get up, there are many little bodies on windowsills through out the house. I expect they come in when the windows and doors are open all day only to die overnight.