Here is the remedy for ragwort. Stop overgrazing.
His article goes on to dismiss docks, thistles, fat hen and nettles as "useless to the human race". However, three species of butterfly lay their eggs exclusively on nettles (small tortoiseshell, red admiral and peacock). The painted lady butterfly lays eggs only on thistles and the small copper only on the dock family.
In addition, all adult butterflies feed on nectar which ragwort and thistles supply copiously. Birds such as linnet, goldfinch, greenfinch, bullfinch and red poll feed on the seeds of thistle, docks and fat hen etc.
Mr Duff Hart-Davis may welcome the elimination of such weeds (and thus their dependent wildlife) but I and my friends rely on these things for some of our spiritual well-being.