We really do need a photo

Yes I agree.
alternatively google images for it your self but try the RHS website to be certain of its identity.

If this property is one you've bought rather than just rented, and you were not advised of the presence of Japanese knotweed, the Agent concerned, and the seller, has broken the law - Japanese knotweed is a 'material fact' that may affect both the decision to buy the property, and the mortgage company's decision to lend the money to buy it, the same way that the building of something like HS2 within five miles might. Many mortgage suppliers refuse to lend on properties which have Japanese knotweed within a certain distance, more information here https://www.onthemarket.com/content/buying-or-selling-a-property-affected-by-japanese-knotweed/