What to do if your neighbour has Japanese Knotweed?
Firstly, you should approach your neighbours and ask them if they are aware; bare in mind that most people do not or may have never even of heard of Japanese Knotweed, no-one should feel the need to panic but to just send some photographs to a specialised Japanese Knotweed company to identify.
Japanese Knotweed is not illegal to have on your property but is illegal to spread onto neighbouring properties or other land, so the sooner the infestation is either removed or contained the better.