They're Nothern Mule wethers.
I think I may well have to put a top fence on the wall, at least as a temporary measure. Grass is getting a bit short - they were due to go to market this Wednesday past but didn't due to the awful weather and flooding - but they've been jumping perfectly good fences for months, turning them into battered messes by half climbing in the process. Up to now, it has been within my boundry so I've just fixed it up and carried on, but they are going onto someone else's land now, so rather more serious. I'm not certain that the boundry fence isn't the neighbour's responsibility, but that isn't really the issue here.