it's not "a fungi" Steve - there's no such thing - it's a
fungus (I know it seems pedantic, but it's the same as someone saying "what is this wasps?" or can anyone identify this trees?"
that said, it is very interesting - at first glance it looks rather like an
Agaricus species (same group as field mushroom and the standard mushrooms for sale in the shops. . . )
but they should have a membranous ring on the stem and there is no evidence of that here - is that one typical?
also is the apparent yellow coloration of the stipe (stem) as it actually was?
I suspect it might turn out to be an
Agrocybe - when you say "I've been unable to identify this one, not even to species" do you then have a genus in mind?
are you able to collect some of this? a small slice (like the slice of a piece of cake) should be enough for an ID amd I would be willing to have a look at it for you
interesting one
best
Chris