If the fence is thiers (and you have checked the deeds to make sure) then providing they are not causing damage then there is little you can do.
I would be inclined to give them one more chance and say that if it isn't fixed by xxx then you will be clearing your area and 'pushing' the fence back to an upright position.
However when you do this you must make sure that you do not damage the fence as this could be criminal damage!
Other than putting your own fence up on your own land there is not a great deal to be done. A second boundry fence however, is not a good idea.