There is a lot of it about - I've come across it in two gardens in the last couple of years but I think there is some growing along the banks of the Wandle, so it will always pop up, and it only needs a tiny bit of root material present to spring up again if its been dug out.
It needs to be disposed of very carefully... its controlled waste so you need someone with a licence to deal with it if its being taken away.
There are a few companies around who can help get rid of it, like this one
http://www.japaneseknotweederadication.co.uk/
Good luck!
www.rosienottage.com