What a fine specimen! We think it is a Deer Shield (Pluteus cervinus), a relatively common mushroom that usually grows on rotting deciduous wood. It’s a match for the picture in the Collins Complete Guide that we use but other sources show a mushroom with a lighter colour so we’re not sure – Pluteus cervinus apparently gets its common name from its dull brown color. All very confusing, and DNA studies apparently suggest that not everything labelled as Pluteus cervinus really is. We should have looked harder at the gills (which should have been free) and and the spores (salmon pink).