My elderly parents have found a house that they really wanted; two days from exchange, Japanese Knotweed started to appear in the garden. They've pulled out as they cannot cope with getting rid of the stuff which will take 3-5 years and prevent them from making a nice garden, and building a garage which will sit over the knotweed area. My father is ill and has dementia, and its just too much for my Mum to cope with.
Needless to say, the developer - who last winter just chopped it down and 'got rid of it' - Lords knows how - had no idea what this plant was. There's no telling how far its spread, and it seems to be sprouting in the adjoining gardens, too.
The developer has basically told my parents to 'shut up' - he's sacked the agents who he said had blown it all out of proportion. He's changing his solicitor as he know about the knotweed, and he plans to market the house knowing FULL WELL that knotweed exists on the property, and has asked my parents to keep quiet.
There are only certain people who know that the property has knotweed, so if the news leaks out when the property is re-marketed, he may well come back and harass my parents thinking it was them that did it. I cannot risk them being involved any more.
The developer says he has taken out a contract with a knotweed exterminator whom my mother had several long chats with; the expert said it would take a minimum of 3 years - possibly 5 - before it was eradicated. It was the knotweed expert who noticed it coming up the the adjoining gardens.
I'd be very wary. We have JK in our veggie patch (well, it will be when we get rid of the stuff). DH treated it last summer with farm strength, not available on open market, killer. It's shooting through right now. He reckons it will be at least 3 years of intensive treatment until we can even think about planting. We did know about before purchase, but in your parents case i wonder if its worth taking advise from The Enviroment Agency?