I had a small shrub growing on top of a wall. I think it was
elderberry. I cut this down a couple of years ago, but it then regrew.
Unfortunately it has quite cleverly distorted the drain pipe, which
has added to the problem, but giving it a steady supply of water! I
have now cut it down and pulled out a rootstock about the size of a
fist. However there are still a few roots that go down into the wall
cavity and I doubt I will be able to remove these.

So, what is the best way to treat the roots, so they don't resprout. I
understand that ammonium sulphamate, which was sold as root out has
now been banned.

Appreciate some tips.
Thanks Tim