thanks all

No easy answer to this one, I'm afraid. You need to cut it back and then dig out the roots, and often that's impossible amongst existing planting. What I usually do in that circumstance is use SBK - I dig around and excavate a bit of soil to expose the woody root, cut into it to expose the interior, and apply a small amount of SBK, then cover with an upturned flower pot and replace the soil. You have to be very, very careful not to spill the SBK on anything else, and not on the soil, which isn't easy because it's like water. Use it neat (despite the instructions) and apply it from a small container (I use the lid, or a medicinal measuring cup, which is much smaller than an egg cup) into which you've put a tiny amount of the fluid. Best done wearing latex gloves

I just keep pulling it out.