If the previous owner of your home has lied to you about their knowledge of Japanese knotweed on the property, or has misrepresented the property by stating that the plant is not on the land, then you may be able to claim compensation for the costs of treating the infestation.
An object to you making a claim for compensation could be losing contact with the previous owner. It’s common for a seller to leave little or no contact details after they’ve sold a property, often because they want to avoid discussing any issues with the home that they’ve avoided mentioning so far. Tracking down the previous owner is an important step in the process of claiming compensation against them.