nettles can be used to make excellent organic fertiliser - place nettle leaves in a bucket or container, add enough water to cover them , leave for 12 days , stirring occasionally. Strain off and you have free feed for your plants, veggies etc.
Thanks for all the replies but they are covering about 15 feet of the garden and if I am away for a week or so I have to "strim" my way to the garden shed!
Weed killer won't kill them forever, however NettleBan will. Best to go to a farm supplies shop as most garden centres don't carry it. It is expencive but used correctly it will do the job. Spray them and leave them to turn black and die back into the ground.
Or you could just keep cutting them back to the clay and that will eventually weaken them enough that they die...