Shearling, spot treat and keep the sheep off it for three weeks or ring fence the treated area. Despite what Monsanto claim there is a suggestion that glyphosate stays in the soil. Some research in the States is suggesting its responsible for increased abortion rates in grazers, this is on land thats been treated for several years, I am no chemist and not organic, but I am loathe to treat with chemical year on year. I live in Fenland and the arable boys throw Roundup parties two or three times a year, its a fact that there is evidence now of some weeds becoming resistant and that worries me.
So spot weeding not a problem for me, you might have to look at re seeding if you cant solve the thistle problem. If its persistant and near a watercourse look at your neighbours nitrate run off, a bit of detective work can pay dividends!