Hi C - thanks for replying,
After reading a number of reviews, I decided to buy 'Mushrooms and other fungi of great britain and europe', by roger Phillips - several quite knowledgeable people said it was their favourite, and I thought, 'good enough for me'. What they didn't mention is that it's pitched somewhat above the level of a complete novice... I'm going to get an idiot's guide to help me read it, next time I get to the shops (any recommendations gratefully recieved).
The mushroom has opened up a bit now (
www.sylvanus.net/mushroom2.jpg), and I now think it rather resembles a parasol; I didn't think it was before, because it seemed rather large to have not started opening. The ring around the stem where the cap was once attached is moveable, sliding up and down the stalk, which (apparently) could be another indicator of Parasol-ness.
No spore print as yet, but there's no hint of greening around the gills (apparently there's a green-spored imposter)... As for the habitat, I found it in a shaded patch at the bottom of a hill in my local wood.