[arugula]

What does 'notifiable' mean??

We have that at my place of work.. 

It means you have a duty to report its presence. You should do this to your council and they should be interested, sadly from replies here they aren't always so.

[Goosegirl]

To be really sure, send pic to, say, RHS. Notify your council whether they help or not, and get it in writing. Whatever it is, try injecting glyphosate if you've only a bit and see what happens.

[Yorkie]

It was killed off on our site by injections of neat glyphosate.  Just make sure you mark the stems you've done with spray paint or similar, or you'll lose track!

[spottymint]

I too have killed knotweed by snapping the stems & filling the hollow stems with systemic weedkiller, took a few goes though.

Injecting the living plant sounds wiser.