It is on the peripheries of the knotweed-infested field boundary that most vigorous spread and growth occurs, as it seeks to reproduce itself.
As this has escaped both into the public domain and onto "live" farmland, one would surmise that these areas will be addressed as matters of urgency.
Some kind of notice informing public of the plant and of its debris conforming to hazardous waste specification, along with a physical barrier(fence, for example) erected by the owner of the source of spread of knotweed...all these would be of immediate and longterm benefit.
These measures, though not "legal" requirements, could work well as a public "statement" of the fact that knotweed has been found and its eradication engaged with in an informed, responsible, systematic and "other-centred", even neighbourly manner.