Lots of folks think there's a law which means people
have to control the ragwort on their land - not so I'm afraid.
There's the Control of Weeds Act 1959 (2003) which makes it possible to obtain an order, to make them control it, but you have to go through the paperwork
Here's a link to info from DEFRA about controling harmful weeds
http://www.defra.gov.uk/publications/2011/04/19/harmful-weeds/I'd definately make an approach to the owner though - at least make them aware (gently) of your concerns.
I have wee bits that spring up from time to time (in between pulling out sessions) and have to say that I've never seen the pigs touch it - I think they know it's not good for them
As for fencing it - a good 12V energiser (or mains if it's feasible) will power 2 strands of white tape around the area with ease - just make sure you've got a good earth connection and regularly strim down the edges to save it from shorting out. Why not consider a 'grazing' pig - Kune Kunes immediately spring to mind
Karen x