Had it in my garden several years ago. Took two years to clear but used roundup but make sure you get the one that specialises in clearing knotweed. It gets into the root system and kills the roots off. Have not seen hide nor hair of it since.
That's amazing. Haven't got any Japanese Knotweed (fingers crossed - I hope not anyway!) - but I do have a garden which is a living proof that Roundup kills f*** all.