I used to work as a journalist on a newspaper in Mid Wales and every winter we would have similar stories running, basically as the wildlife hibernated in the forests, the cats had to hunt and you'd have a reasonable number of sheep being killed for meat by what was assumed to be big cats.

The RSPCA guys I talked all seemed to think that it was a perfectly plausible situation, as the bigger cats can easily cover larger distances, and the attacks were different to those by foxes.

The most interesting one I had was when there was a reasonable layer of snow on the ground, and the size and spacing of pawprints were clearly something bigger than a fox, and the attack was pretty gruesome, although I expect fox attacks would be gruesome too.