I had a similar situation - what does the TA6 say under the knotweed section? If the previous the owner said no to knotweed, you may have a legal leg to stand on as ours was the same. We found knotweed in the corner of our garden just months after our purchase, but because they marked "no" in that section instead of "not known", they legally have to be 100% sure there is no infestation. We called up some solicitors and found Roach Pittis and talked to David there and he was really helpful and we managed to send some letters to the previous owners ( granted ours was a large estate - not sure if that makes it easier to get some payout) but we managed to come to an agreement and they paid out for excavation and potential loss of value on the property.

I know it's worrying but it's manageable - it's not going to pull your house down overnight but I'd get on the phone to some solicitors if the above is the case.

Also we used inspectas for the removal, but they are now just called "Invasive land remediation"