I believe this to be a flesh fly - Sarcophaga carnaria, Length 13-15mm. I photographed one of these in my garden last year, in August, and I've seen it again this year in my garden. Your photo is much clearer than mine though.
What my book says:-
Occurs commonly and abundantly in the vicinity of man. Frequents both flowers and excrement. Females avidly seek meat, in which they lay their eggs, these are also laid in all sorts of food remains. An extremely unpleasant species, particularly in the south, Distribution: Europe, Africa