your moss is almost certainly Polytrichastrum formosum (a.k.a. Polytrichum formosum) one of the more robust upright growing British mosses. It is well adapted to growing in rather acid environments and this may be why it laughs in the face of your Iron sulphate, liming might be more effective. Because of their different physiology many of the weedkillers commonly used eg. Paraquat, etc. don't kill mosses but I believe there are moss-killers available for those who prefer putting greens to nice diverse communities.