Winter Aconite (Eranthis hyemalis). Harbinger of summer sunshine, an ancient introduction from the continent. Grown in gardens they escaped and naturalised in the countryside here and there. 

 Available as a bulb in autumn which is probably the best way to start them off. I also see them sold in little pots at this time of year. London clay doesn't suit them; they favour chalky, well drained soil. I have a few in an old china sink in my front garden in London but I know of several patches in Herts where they run wild.

 E. hyemalis is a species of open deciduous woodland in its natural habitat. The flowers unfurl on sunny days when the dappled light filters through the bare branches.