I understand from others that it requires more than 1 application of
weedkiller. I have seen some that was treated with weedkiller, cut down and
then covered with tarmac. It popped up again the following year. It
regenerates amazingly well from the smallest piece of root!
Gardening programmes always advise that the only really effective way of
getting rid it is to dig up the roots, ensuring that you remove every tiny
piece of root - clearly not possible in this instance!