Thanks for the input guys.
I’ve still not managed to make a positive ID, but I’m hoping to get down that way again soon. I’ll record more information about the local environment this time, and also collect a sample.
I think there is an ash tree close by, so they probably are ash leaves in the picture.

is a species I have never seen myself, but the best idea I can come up with.

Not Hydnellum of course, they have brown spores, and would have visible spines at the stage of maturity anyway.
I was also thinking of Fomes fomentarius, but that species releases its spores in spring, not autumn.
It’s too thick and greyish to be Heterobasidion annosum.
So, there aren’t many left with that size and white spores to choose from. Isn’t there debris and leaves of ash in the pictures too?

It might be a ‘conk’ of some sort, but that generally requires wood of some sort. It also might be a young Hydnellum (I think those have white spores).