The field blewit (Lepista saeva) is commonly found in pasture but, despite its name, also occurs on heathland and in broad-leaved woods. It likes alkaline conditions and poor soils so the limestone Purbecks cliffs suit it. Widespread and common where it occurs it has a tendency to form in rings although sometimes they may just be in random groups.
It has a large, waxy cap up to 12cms across to it is a big fungus and it is, apparently worth eating. I will leave you to try it.