I saw this fungus on a fallen tree in Colinton Dell (where much of the dead wood is allowed to lie, offering great places for fungi and lower plants to grow and invertebrates to hide away!). It's very noticeable with its large brown toadstools.
But less noticeable is the network of dark thread like structures you can see in the photo above. This is the mycelium and is present all the time, even when the toadstools have died back. The toadstool is the fruiting body and in many species is only present for a short period of time, until it has shed its spores at which point it dies back. The mycelium on the other hand, endures! Many fallen trees are full of mycelia.
I'm not sure what species this fungus is, perhaps velvet shank or brown stew fungus. If anyone can recognise it from these photos, please let me know in the comments!