Isn't it a pity you can't get a gun attached to a camera which can be controlled from anywhere, so then you could shoot the dog in question. Has anyone around where you are, ideally nearest to your land, have any big dogs? Did you talk to the authorities? I would suggest finding the owner, if possible, and bill them for the sheep and request they have their dog put down. It is irresponsibility to leave a dog permanently on the loose. Are you able to get DNA tests done on dog poop, or any poop for example?