Why they are deemed a problem? Ragwort is poisonous for horses and cattle, although not it seems for the deer at ELF. Humans can also suffer from cirrhosis of the liver if it is handled sufficiently.
The legal case. The Weeds Act 1959, amended by the Ragwort Control Act 2003, aims to prevent the spread of Common Ragwort. It provides powers for the Secretary of State, any person acting on their behalf or the Secretary of State for Environment, Food and Rural Affairs, to prevent the spread of Ragwort on private land, which, if not adhered to by the owner of affected land, can lead to a fine up to £1000 and further punishment.
and Code of Practice on How to Prevent the Spread of Ragwort (PDF)
Although only one of five plants listed in the 1959 Weed Act, ragwort is regularly rooted for removal. The reasons are, arguably, a) it is more toxic, b) more noticeable with it's bright yellow flowers and c)is more able to spread further afield with it's parachute headed seeds.
.... when tested in a variety of conditions, 31% of the seeds travelled only 1 metre, 89% of them 5 metres or less and none were collected more than 14 metres from the source. The study involved studying the dispersal of over fifty three thousand individual seeds. ragwortfacts.com
The management on this land.
The ragwort is pulled up and bagged. Care has to be taken in not leaving viable root parts. SeeJuly 17th 2015.
The original fields, those to the south of the main drag, seem all but cleared of ragwort, so there is some relief that all the effort worked and is still noticeable.
Most of the weed is pulled from the bottom of the Triangle Field; 90% of which is the area to the south of the path to the Far Fields.
Work diary: Pages (3 ~ 2013) (4 ~ 5 ~ 6) (7 ~ 2017)
The marsh ragwort looks like a healthier version of the common ragwort, slightly taller with bigger leaves and larger flowers. It is also just about as toxic as the common ragwort but is not statutorily controlled. However common sense says that if it became a problem then there is likely to be liability.
However as it is found in the marsh, where no domesticated animals feed, not that we would have them, and there is little or no chance of the marsh ragwort spreading to neighbours fields, it is not curtailed.