With my new dynamic self.. time to take on that knotweed again. Little bit of interweb research (wikipedia > search ‘japanese knotweed’) 
I popped off to B&Q to nab me some toxins.
Back home with my box of fun (hopeful plant death), I set about in my alchemy lab (kitchen) to concoct the vial brew (put the disolvable plastic sachet into water, whilst trying not to eat it).
To my Joy (litterally GLEE) it turns out I have managed to purchase “Alpha-betty-weedkiller” !!
Lookit!:
Seems all the lettering off the disolvable sachet came up to the surface! what fun!