Thank you, Alan.
I will put this forward as probable C. pseudoniveus, then.
It was a very, very tiny specimen indeed - only about 3 or 4 cm high at most and looked far too cute to even consider picking, especially as there were no others in sight (and I got sidetracked by the Red Arrows flying over on their way home after the nearby Kemble Air day!) Definitely in the very early stages of its visible life!