As an aid to understanding what might have been going on in the Dart for the past couple of years, the following link is to an abstract of a paper that gives a more detailed pathology of UDN.
Ulcerative dermal necrosis (UDN) in wild salmonids
There is a suggestion that the disease we have been observing may not be UDN. However, the article above appears to describe precisely the appearance and consequences of what we have been seeing.
The good news is that at least one shoal of clean sea trout has been spotted in the river recently.