I feel sure it's this one, at the bottom of the page on next link, Melanostoma scalare.
http://www.gardensafari.net/english/hoverflies.htm#overige
The write up says males often have green halters, and yes mine has!
Difficult to find a pic of the underside, as that's all I got. I found one which does show some white under the abdomen, mine also has a couple of square patterns.
If there is nothing else comes close with these two features then it has to be Melanostoma scalare, the legs look to match too.
http://www.fugleognatur.dk/english/gallery_uk.asp?mode=ShowLarge&ID=6302
