Blewit

I think this is Field Blewit Lepista saeva; quite an old specimen, cap 50 mm. 

from a garden at Saville Park, in the soil at the foot of an old privet hedge. Found 8.1.15.

I usually find Wood Blewit Lepista nuda in gardens, which I have occasionally taken home to cook, but as they often come up in compost heaps and grass-cutting dumps, and these are quite likely to have had garden chemicals incorporated, I  now tend to leave them alone.