A Dark Greenbottle Fly?

What I can only assume is an unusually dark greenbottle fly, possibly Lucilia sp. (Diptera: Calliphoridae), on a thistle (Cirsium sp.).  The only thing like it that I can find out on the web is Lucilia silvarum, but they appear to be much brighter and cleaner than the above specimen, so I won't claim this as an identification.

Photos taken beside the Aire and Calder Canal, near Rothwell in West Yorkshire, on 2009-08-03.

Update on Thu 2010-02-18 by Tristram Brelstaff

A post here at wildaboutbritain.co.uk suggests that greenbottle flies do tend to turn bronze as they age.