>as we were miles from the nearest house and I think an escaped dog would have had the sense not to walk half way up High Street by itself in that weather.
They could have escaped a while ago and been living in the area.
Basically any scenario where you can imagine a big cat showing up a dog could do the same thing, probably more likely to as wolves at least were native to Britain.
Hmm, semi-wild dogs seems less likely to me as they'd have to hang around settlements to get food, in which case they'd get noticed and dealt with. Unless you mean they can hunt in packs and take down sheep?