A nasty looking fly!

I was in the garden again today mostly weeding but occasionally chasing butterflies and hover flies with the camera.  This fly attracted my attention although for all the wrong reasons;  I just didn't like the look of it!  I haven't been able to positively identify it from my Collins Guide although it does look similar to a Tachina Fera.  It has, at any rate, encouraged me to order a copy of Brock's Insect Guide (as recommended by blipper Knottman) as I do like to know what I have photographed.