Make sure you don’t have rubbish lying around outside or the compost too close to the house as this is perfect breeding grounds for flies. Also, if you have a dog make sure you clean up after the dog regularly as this also attracts flies.
We also supply fly catch bags. We hang the catch bag from a tree or something away from the house to attract the flies to the catch bag. We get great results with the catch bags as they can catch hundreds of flies