we are at our wits end having tried a number of suggestions none have worked and this has been going on for a number of years, hopefully you can resolve the problem as we are out of ideas.!!!! thank you for your time and trouble.

It sounds as though they're in a dry spot, and struggle when the summer new growth is put on. Is the first flush of spring growth alright? If so, that would indicate to me that the hedge is surviving, but not thriving, and when the (generally) dry weather comes along in summer, dryness at the roots causes the growth to be weak, and prone to the mildew, for that's what it sounds like. Good soaking in June/July, and a mulch to conserve the moisture. See what happens then.
Phil J