I haven't tried to kill it, I just pull it out where it isn't wanted. I would have thought that it would be susceptible to weedkiller containing glyphosate.

Thank you for your advice. I've never used weedkillers before as I've never had such a big, untamed garden but I will check these products out at the garden centre. The root going under the house is about 8cm diameter so it won't pull out and it's in an awkward place to dig. It's also popping up all along the edge of the brick wall of the garage. I used to think it was a lovely plant- not any more.