Japanese Knotweed doesn't form viable seed in the UK spreads by rhizomes and cuttings, are you sure it wasn't giant hogweed.I remember watching something a couple of years ago, something like A Year In The Highlands. They looked at how wild beavers were thriving on Japanese Knotweed. Unfortunately, rather than suppressing it, they were spreading the seeds downstream in their spoor.
So there you go, Jock, eat Beaver sh¡t.
Memo to self. Need a