So, the blood test was negative, much to the vet's surprise. Even though the test has a very high false negative rate, the vet had thought that because the ewe showed clinical symptoms, it was more likely that the blood would come back positive.
We had the abattoir vet do a PM and they found nothing. Not the characteristic thickening of the ileum - which again, absence doesn't mean it's not Johnes but presence is diagnostic - but she also found no other problems which could explain the symptoms : no tumours, enlarged organs, nothing.
So, because we really want to know, of course, for knowing what if any management we need to do with our younger sheep and our new young cows, we had collected some faeces before we sent her off, so that we could have a faecal test if the abattoir PM was not informative.
The lab had said that in the circs - presenting clinically, serology negative, PM negative - they thought that the much quicker PCR faecal test would be accurate enough, and that if it also came back negative, then we could be pretty certain that it wasn't Johnes. So that's been done and was negative.
Which is great news for the rest of the flock and the young cows, and it means I can relax my mental gymnastics working out who can safely graze where and when, what piles of pony poo are safe to be spread back on the fields, etc, and means anyone can graze anywhere over winter and next spring, which is a great relief.
But as to what was going on with Gwenneth... we will never know. We can keep her good daughter on, but it's very sad, she was a good sheep and the last pure Zwartbles here.