I have just noticed a tall vigourous plant that I thought might be Japenese Knotweed. It has a similar stem (fleshy, hollow, in sections with red tinge) but the leaves are much smaller, spear shaped, with serrated edges. It pulls up easily and has very shallow stuuby roots that area pinky/purple colour.
While I am confident it is not Jap Knotweed, I am concerened that it is an invasive species that could cause problems. The area it grows is very wet, really a lake that floods up in winter and dries back in summer.