If your neighbour has Japanese Knotweed, then you should tell them as soon as possible. If they do not arrange to have the Japanese knotweed treated and subsequently allow the Japanese knotweed to spread to your land, then you may be able to bring a claim against them. Currently there are no strict rules that you must inform a commercial neighbour, especially if there has been Japanese Knotweed present for years as it is fair to say that they should already be aware.