Over 40 years ago we identified some in our back garden. At the time you were advised to use a particular weed killer. It didn't work.
We gently dug a quite wide pit extending below the roots. We retained any plant parts that got separated from the main body.
Finally we lit a bonfire in the pit and burnt all the plant parts. We kept it burning for several hours.
There was no further problem.
I did follow the link that was up thread but the site wasn't particularly user friendly so I don't know if this is forbidden.
Also there were only seven or eight plants and no overhead foliage but it certainly beats putting all those poisons into the environment and paying out pounds for an expert.