I returned to the brown mushroom from the other day. Looks to me like all the mushrooms are the same species, but they are varying sizes, and were in different stages of growth the other day. I took a few shots from varying angles today, including one of the gills. Looking at these a few days later I suspect this is Flammulina velutipes. There is quite a lot of variation in the pictures labelled as this species on the net.