This fabulous fly is Tachina grossa (Diptera: Tachinidae), which belongs to a group of parasitic flies. In this case its host are the caterpillars of a few species of large moths.
I always think that their flesh coloured head makes them look a bit like the fly with the human head in the Vincent Price film.
Not sure where you live David but I'm glad the house flies around here aren't that size :-)
Click on the photo to enlarge it and you will see the more distinctive colouring and other characteristics.