Strong weed killer - what's the best?

I need a strong weed killer to spray between the back of the greenhouse and our next door neighbour. This is only a narrow 2 ft gap so digging out is almost impossible. The house next door is owned by an elderly lady who cannot get out now so the garden is just a m of weeds and brambles all of which are now coming through to us. There would be no danger of any chemical getting near the plants growing in the greenhouse as there are two foot concrete slabs that end of the greenhouse holding some staging. Is Jeyes fluid strong enough and if not is there anything else I can use that will be effective. I live in England.