Currently trying to restore a garden which has been left wild for years.
There are loads of brambles - some of them have huge stems of wood 3" in
diameter so they've been there a long time.

In order to dig out brambles I've read somewhere that you only need to sever
the roots below the point where the new shoots emerge, which is at the
knobbly area where stems meet roots, similar to that area of a grafted rose.
The cut roots left below this area will rot away and not produce new shoots.
Can anyone confirm this?

I don't know if it's always true, but that is my experience.