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The liability for cover would come from your insurer at the time, so in 2015 when you had cover for the property which now has the knotweed. If the insurer is now refusing to pay out you need to counter sue them. I do not know how far along you are with court proceedings but you can counter claim against the insurer or join them in to court proceedings. I would also recommend making a formal complaint to them and then escalate to the Ombudsman at http://www.financial-ombudsman.org.uk
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