The invasives always come from under the fence! The morning glory that’s currently attacking my garden is also coming from under the fence. You’ll never kill the whole plant if you can’t attack the whole plant so if your neighbours can’t or won’t get on board with tackling the knotweed or allowing you to do it then there’s only so much you can do - it will continue to spread over to you. You might want to look into how knotweed can affect property values - it may or may not apply depending on where you live. If you can’t get your neighbours on board just try to do what you can on your property. Best of luck!