What does “Not known” actually tell you?
The seller is answering a formal enquiry from their own knowledge. Where they answer “Not known”, they are recording that they have no reasonable basis for saying either yes or no — perhaps because they have owned the property for a short time, have never inspected the boundaries closely, or have no information about what happened before them.
That is a different thing from a denial. A “No” is a representation that you can rely on as a statement of the seller's position. “Not known” makes no representation at all about the plant. Nothing has been ruled out and nothing has been admitted.
It is worth saying plainly that “Not known” is not evasive in itself. A seller who does not know and says so is answering the form correctly. The difficulty it creates is practical, not moral: you are left without the information.
Is “Not known” worse than “No” for a buyer?
In one sense it is more useful, because it tells you accurately where you stand. In another it is harder, because it removes the fallback that a denial would have given you if knotweed later appeared.
It also tends to matter for the responses available to you. Some risk-management options assume a negative answer has been given, and an express statement of uncertainty can change what is on offer. That is a question to put specifically rather than assume either way.
What should happen next?
The question moves to inspection. Ask your surveyor to comment specifically on Japanese knotweed and tell them the seller has answered “Not known”, so that they are looking at the question rather than assuming it has been dealt with.
Your conveyancer can also raise a further enquiry asking what the seller does know: how long they have owned the property, whether any treatment was carried out in their time, and whether they are aware of knotweed on adjoining land. An answer of “Not known” to the main question does not prevent more specific questions being asked.