Posted: 10/08/2015 at 22:02
It looks quite small for japanese knotweed. That may mean the sellers have chopped it all down and treated it once already. Did the survey show it up.? if the vendors knew about it, it should have been declared.
However... I would spray it all with a glyphosate solution. It should turn brown and die. You have to burn the residue on site. You cant put it in a bin or move it off the original site. If it regrows, spray again. It may take a number of sprayings to be rid permanently.
Is that the house wall or a garden dividing wall? Is there growth on the other side of the wall? if so, that will need treating as well.