Posted: 01/07/2014 at 18:04
Hi Andy - treat them with a weedkiller containing glyphosate and then leave them there to die.
The glyphosate will travel back into the roots and kill the whole plant. It will take two or three weeks.
When it's all brown you can cut the nettles off flush with the brickwork.
As you so rightly say, if you pull them you run the risk of disturbing the mortar.
How conveniently close you are to the village pub