I have seen what I think is a clump of knotweed growing on a cycle track behind my house. We have been here many years and have never seen it before anywhere in this area. Does it have to be reported to someone, since the track belongs to either a council or county council, not to an individual.
Phone your local authority and ask for the environment department.
They will have special procedures in place to deal with it.
Asking the name of the officer involved may also prove useful.
Did you know Jap knotweed is edible. In spring, cut the new stems down and then cut into 1" slices. Stew until soft in a sugar syrup. Taste just like rhubarb. Saw it on spring watch.
To kill the stuff, cut it down to 18" and pour a strong roundup mix into the hollow stems. may need several goes. Do this in late summer.