Red fox mostly live on mice, voles, worms, rats, bugs, road kill, small snakes, and lizards, and the occasional rabbit or bird,
If seen in a field thick with spring sheep, as here, they are mostly likely looking for afterbirth, already-dead lambs (an astounding mortality with newborns!), or testicles that have fallen off after banding.
Notice that the sheep here are as relaxed as can be.
We get the same thing here in the U.S. with red fox, but it's a different matter entirely when coyotes show up. Coyote are larger and will hunt in pairs or small packs, and they can occasionally tip the scale at 40 pounds, as compared to 15 pounds for the average fox. Even with coyote, however, sheep predation is not a given, and seems to determined, in part, by the amount and nature of alternative food sources available (such as roadkill and garbage).
That said, coyote predation is real. The latest news out of Southern Virginia is that the feral hog population at Back Bay National Wildlife Refuge down near Hampton Roads is getting "managed" by coyotes. Excellent!