The seller of a property is obligated to ensure that any information they have provided to the buyer is kept up to date, right up to the day of the exchange. This means that if the seller initially reports, truthfully, that there is no knotweed present on the property and then later discovers an infestation, they are obligated by law to inform the prospective seller. There is a stipulation in the standard TA6 Property Information Form that requires the seller to inform their solicitor in the event of a change in the answer that they have initially provided.  Failure to communicate this change could lead to a potential claim against the seller for misrepresentation.