The message
from "Brian" contains these words:

Japanese Knotweed can become a major problem if the eradication process is
not sustained~~ even though I have known it to take four years with
Roundup[The best]. Leave it untroubled and your only solution is to move
house!!

And that's despite the fact that it doesn't spread by seed in the UK.
The nation-wide infestation is supposed to have originated from one
plant. It can erupt through concrete :-(

One of my sons moved into an Edinburgh tenement flat where the small
walled garden was solid, head-high JK. It took four years, but we
managed to eradicate it by first, chopping down and burning all the
plants, then cutting off and destroying every single new stem as it
emerged, **before it could develop any new leaves**.

Janet.