https://www.planttracker.org.uk/ is a website where once you’ve registered you can report Japanese knotweed.
The Environment Agency, Scottish Natural Heritage, Natural Resources Wales and the Scottish Environment Protection Agency have joined forces to help combat the spread of the UK’s most problematic invasive, non-native plant species.
It’s easy to register on the site. You do need a photo of the knotweed you’re reporting too.
I’d recommend you report any you find near you as it will encourage councils to take action where it’s on their land and reduce it’s spread.
I know there’s some on the brook near me and I will go and take a photo of it in the next few days and then report it! I had taken a photo when I’d originally seen it but it’s not very good so I’ll have another go.