My dog is safe in my garden on a zip-line?

[Ms C]

A man walks his dog off lead along the back of our fenced in garden. His dog comes up to the fence all teeth and growls and sounds quite viscious our dog responds the same way with hackles up. We've put a stop near the end is now about 4' from the fence and the off lead dog has started to dig his way in, , we've blocked up the holes so far but this dog is quite persistant and all the time my dog is going crazy at the same time. my question is if this dog gets in and my dog kills it (because by the sounds going on he will) who is at fault? the owner just watches, makes no effort to control this and as it happens twice daily so it seems to be ok with him