I have just posted a feed in "Sheep" as I have an SSSI, which is for chalk grassland and orchids (5 species recorded, including bee orchid, pyramid orchids and fragrant orchids)
Natural England have just been out to see me and have suggested that I put Hebridean sheep on the land (Quarry bottom - 4 acres) to help keep the orchids and grasses healthy. They said that sheep do not harm the orchids and would in the long term be good for them by clearing the site and keeping the grasses down to allow the orchids to prosper.
They are going to let me know the optimum number for the area so that I can keep them there all year round.
I don't understand this fully as yet, as I have only just acquired the land, but this may help in your query. (Hopefully)