Introduced into Britain by the Victorians (thanks for that!) Japanese Knotweed is extremely invasive.
It can grow up to 100mm in a day in the summer months. Concrete, tarmac and brick are no barrier for it.
It is recognised by its shovel shaped leaves and bamboo stem and white flowers in autumn. See Photos below.
If discovered you should:
Create an exclusive area of at least 20 feet from the plant.
Spray it with a glyphosate-based weed killer.
Cut it down and dig up the plant, including the root and burn it within the exclusion area.
Wash feet and clean foot wear when you leave the site.