I have found a circle of fungus on my lawn and having done a bit of research I think it may be Marasmius oreades.
The grass is much darker before the fungus appears. When it does appear it appears in a circle about 4 to 5 feet accross under a tree in my lawn. There are lots of spent leaves and buds from the tree that the fungus grows through. They grow in quite compact bunches, with the individual bunches combining to form the larger circle. They are waxy/slimey to touch.
I've heard that Marasmius oreades can be confused with Clitocybes. I'm happy to let the fungus grow if it is the Marasmius oreades!