I think it's horse tail; dreadful stuff, very, very difficult to eradicate once it's established
I once bought in some 'sterilised' top soil for some raised beds (ten years ago) and I'm still dealing with the horse tail that came with it!
Why didn't I think of that haha. Thanks Baggins!Looks like a potential Tawny Grisette to me to.
Just a useful hint. If you want to get a pic of the underside of fungi without disturbing it, use your phone, flip the camera into selfie mode and hold it under the cap. You can then use the phone screen to have a look or take a piccy.