Please tell me this isn't Japanese knotweed!

Could someone please tell me what this plant is?  It's been hiding behind a particularly manky shed in our garden (which has just been demolished) and we can now see it in all its (dubious) glory.

Many thanks for any help!

Our herring gulls are red listed birds.  Think about that the next time you hear some flaming idiot calling for a cull of them.