I would suspect it is more likely to be one of the Flesh Flies (Sarcophagidae) than a Blowfly (Calliphoridae).
But either way, a good shot.
It was a toss up for me with my limited knowledge.
I have been using my 18-55mm lens as a prime lens (leaving it at 55) and using the tripod as a mono to regain whatever skills I may have had as a bug hunter. My theory is that if I can get a decent close-up then when the macro lens is attached it should,'t be quite as steep a learning curve?
As a very rough guide, Brian. Muscidae and a few other groups which include the 'Houseflies' and quite a few others have wing veins which run full length to the wing tip. Tachinidae and Sarcophagidae (Flesh Flies) tend to have a very distinctive bend in the long veins towards the wing tips. Tachinidae often have a very spiky rear end.
Which tends to leave the 'Blowflies' in between those two groups but usually any wing veins aren't as distinctively angled and they often have shiny bodies, or parts such as the abdomen are shiny.
But unfortunately the correct division between these groups relies on small variations which often aren't visible from photos.