Cut away as much as possible and then when regrowth starts use a brushwood killer on the leaves.

ive been digging down as far as poss can for the roots and some dont come back,im going to try scotsgrans suggestion

cut down the topgrowth, dig around a bit and reveal the woody, knobbly top of the roots - now drill into it with a wood bit, not right through, just to make holes, fill the holes with SBK brushwood killer, cover with an upturned pot or something, replace the soil around the pot. Don't spill the SBK on the soil.