I was mowing our 'meadow' (probably more accurate than calling it a lawn!) when I spotted this in a dark, protected corner. It was big - about 5ins in height and 3.5ins across at the base of the cap (see next photo with a 3.5in/9cm pot alngside for comparison). A coouple of days later the cap had flattened out and was about 5ins across and the whole thing had darkened.
If anyone can tell me what it is AND if we could have eaten it, I would be really grateful.