Interesting Sue, do you have any other shots of the gills etc? What is the substrate, wood, earth or other? Was it alone or were there others close by? Sorry for all the questions. It is an older specimen, but I just can't quite place it from this single image, sorry. Teresa
Thanks for looking at this, Teresa. Unfortunately, I only have other photos from above, but I've uploaded them anyway, in case you can spot something. There were 4 or 5 growing in leaf litter underneath a Port Wine Magnolia hedge.
Sue This distinctive fungus is Agaricus rotalis. It is becoming more common over recent years. Often found growing in garden litter. I think it might be an exotic, though have to chase this up. You've captured the cap features well.