My suspicion is that poor husbandry is to blame. The first affected flock belong to an elderly farmer who, despite best intentions is very obviously struggling. He has land all over the place and is spreading himself pretty thin. The second neighbour affected is , shall we say, non conformist and I imagine his sheep are pretty much left to themselves despite him saying he checked them.
We are lucky that our sheep are directly outside, our house is in the middle of our fields, and I spend a lot of my day just watching/observing my flock. Even though I'm such a newbie I would like to think I would spot any issues fairly quickly, just because they are right there more than anything. My first reaction to hearing all this was exactly as you have responded, it cannot simply be a case of mites, it's just not possible. So I'm hoping that my approach to watch and wait is the best one. If I didn't know about others losing sheep, I would have zero concerns about my sheep, they are exactly as they have always been.
Thanks for the advice all, much appreciated.