[cloddopper]

Check with your local environmental health people .. something tells me in the back of my head that they have some useful input. Perhaps access to a Euro fund and chemicals ..

 One way I've recently seen by some organic /green gardeners on a TV prog about hitting it hard .
Is clean cutting the stem about 18 inches from the ground and filling the hollow stem with a strong glyphosate solution using a killer spray pump up pack with it coming out of the lance minus any jet attachment ..

Agricultural merchants sell 5 litre packs of a much stronger content far cheaper than the likes of the Big garden centres ..... friendly farmers can be most helpful .

I'd give this a twirl :-
Present the seller with an information pack about knot weed and offer a much lower price to reflect the problem .  If they don't play walk away .

 Once you have seven years on the holding without using glyphosates & listed chemicals  etc you can call yourself organic .. sounds good to me
 

 I noticed a few weeks ago on my way past Llangunnor Police station that the LA have treated four or five large road side patches .... possibly in this manner . What ever they have done it's certainly kicked its butt hard .. whether it kills it in one go is another matter .

[pgkevet]

You can buy concentrated 5L glyphosate on ebay too.
When I stupidly used willow poles for an espalier frame and they rooted I painted any shoots/leaves with neat glyphosate. It took a few goes over half a season to turn them inert.

If knotweed is hollow then I'd have thought injecting the stems would be better than cutting them and having live material waste to deal with....?

[cloddopper]

It was evidently quicker and just as effective to use the pump sprayer less the nozzle to fill the hollow cut stem than to try and use a small bore tube to inject the Roundup into a complete undamaged stem .