Opinions vary on how environmentally friendly it is, but use it sparingly and I doubt you'll be up there with clearing the Amazon rain forest in the league of crimes against nature.
More weeds will grow there given time, whatever you use, so even if you finish off these plants you won't be able to forget about it forever. You can use boiling water, salt, or a weed burner and it will kill them as well as any weedkiller, but without poisoning the local wildlife.
I have some weeds growing out of the brickwork at the bottom of the house, various varieties.
I want to use the strongest possible, yet cat and environment friendly, product to kill them dead.
What should I buy?