Hi madkipper
That's not easily identifiable to species from that picture. The genus is certainly Agaricus, which are the relatives of the standard shop and field mushrooms. There are some well-known and relatively easily-identified members of the genus, but a lot of others also. Smells are important, as is yellow staining, especially near the base of the stem. I'm not sure what you've got there. It might be A. augustus, but it also might not...
Geoff