I found this mushroom growing by Sainsburys. It was surprisingly easy to identify because I noticed it turned blue where touched, especially in the stipe. Just that information alone was helpful; there are not many mushrooms than bruise blue when handled.
It's called Psilocybe cyanescens/Blueleg Brownie. It's dangerous to eat this mushroom.
Habitat It grows on saw dust and wood mulch and whilst it was uncommon, the increased use of bark mulch has allowed the fungi to also increase in numbers. From late autumn to early winter.
Cap 2-4cm across, convex then expanded and irregularly wavy which can be seen below. The photo also shows the cap is hygrophanous, (it changes colour when it dries). The cap on the left shows an ochre cream colour with a darker reddish edge. The darker colour can be seen on the right.
Flesh whitish, tinged bluish in places.
Gills adnate; pale ochre at first which become dark brown with age.
Stem 25-50mm high/5-8mm wide; white, bruising strongly blue or blue-green. This colour change can be seen below. I've enhanced the colour on these two photos but in real life the colour change is clearly visible