Having looked at a lot of dune fungi over the last few years, I am trying to think what I would think this was if I walked past it – and whether I would stop and take a closer look. Somehow not so easy with a photograph…
I’ve decided I would think it’s Clitocybe agrestis. Very common on dunes, but under-recorded. More or less white, but variable with age and weather. Thin-fleshed. Not very decurrent lamellae. Almost completely featureless microscopically. Stipe smooth but initially may have some cobwebby mycelium. Best guess. If it’s not that, I ‘ve no idea.
Tricholoma species (which this is not) can occur in dune slacks with Salix repens creeping willow which, from the point of view of an ectomycorrhizal fungus, is just as good as a tree-sized non-creeping willow.
I recall seeing the Fomitopsis on sea buckthorn at the pitiful remnant of dune at Minehead – a thin strip squeezed between Butlins and the beach, but still harbouring some marram grass and a few associated fungi.
Keep searching the dunes! Interesting place for interesting species.