Darkish brown solid fungi with edges a slightly different colour. Underside pale coloured gills. Changes to a pale colour with a slight brown tinge on the cap as it gets larger. The gills remain the same colour.
This is appears to be a very interesting record Fiona. At first I thought it might be a Pleurotus, however the gills (lamellae) don't appear to be decurrent (running onto the stipe). Also it is more 'bell shaped' than what I would expect with a Pleurotus. I'm wondering if it might be a Conchomyces sp. possibly C. bursiformis
If you keep an eye on it and if it changes as it matures more that would be useful to record. What diameter was it please? Having a closer look at the stipe the lamellae do appear to be slightly running onto the stipe. A puzzle. Good record.