Why is my plum tree covered in flies and how can I get rid of them?
Look like blue bottles and horse fly. I've had a good look around and can't see anything that would attract blue bottle.
Having googled the problem it's not uncommon but no body seems to have any reason why its happening.
Do you have a recently landscaped garden nearby, a farm or a big pile of farmyard manure.
Blue Bottles and horseflies etc occur where you have a large quantity of manure or decaying rotting vegetation or even worse meat or a carcase and the flies are sheltering in your tree.
Are the plums rotting on the tree?
Because of the volume of flies involved there doesn't seem to be an effective way of getting rid of them except wait until the problem resolves itself on its own whatever it may be that is causing it.
If the plums are not rotting on the tree then I would look a little further afield for the root cause of the problem. Manure, Garbage, Composting bins, a dead carcase of an animal such as a dear or cow somewhere. Is your tree near a stream or river? Is there a carcase in this stream / river nearby. A building site will buy in topsoil for the gardens, it may contain tens of thousands of fly grubs that are turning into flies.
Just a few ideas, look a little further beyond your garden and the tree for the reason behind the problem.