I am very aware that cup fungi like these really need to be confirmed by microscopic examination of their spores but, sticking my neck out and assuming the most common species is most likely the one I’ve found, I’m going to say this is Common grey disco (Mollisia cinerea).

As they are tiny and grey and the white margin to their cups is often not particularly obvious, it is easy to overlook these little beauties. I wasn’t looking for fungi; I simply got lucky while poking about amongst some rotting logs in a local park.