I think I can almost definitely say that it's not Japanese Knotweed - Japanese Knotweed leaves are slightly more triangular and less heart shaped, they aren't as glossy, are a little paler and the stems should be a pinky/red colour. Hope this helps!
We have lived with Japanese Knotweed for a year and a half now and as per the conditions of our mortgage are under a programme or eradication from a reputable company. It's very tempting to go to it and attack it yourself - there have been so many times I've nearly lost my temper and tried to annihilate it myself. But in truth, most local authorities and mortgage companies now will only accept and recognise treatment programmes from registered/professional eradicators using certain chemicals and you'r certainly not allowed to illegally dispose of it anywhere. We've been warned not to touch it with anything - not to cut it or burn it and to live with it until it's the right time of year to treat it again with the correct licenced weedkiller - it will take years to fully eradicate it properly, which is going to take a lot of patience, but will be worth it in the end. It's a nightmare!!!