I came across a few examples of this tiny parasol growing in patches of moss on Cleeve Hill - it must be a fungus of some kind. It’s a very pretty translucent white fungus, taken on macro, so about 1 cm across, with a parasol that looks like it’s made of petals rather than pleats. So far I didn’t find any photo quite like it on the internet. The nearest comparison is Pleated Inkcap, of which there were plenty growing nearly, but these are bigger with a brown centre; and they tended to be found more in grassy areas on the hill.