Posted: 16/04/2016 at 20:07
Nick, it's Ground Elder. Wait until you see flowers beginning to form then spray it all thoroughly with glyphosate. You must then wait for it to die down completely before removing the dead leaves and stems. If you don't wait, the roots won't be killed.
Unfortunately, as it is coming in from your neighbours garden, it will come back through the boundary so try to persuade them to do the same on their side at the same time.