unknown spring plant

unknown spring plant

Observation - unknown spring plant - UK and Ireland. Description: A large patch of these plants were covering a bank near an urban river. The park they were in contains some ancient woodland but these were more out in the open, between a mown area and a patch of gorse. I tried Rose's wildflower keys for woodland and fo

A large patch of these plants were covering a bank near an urban river. The park they were in contains some ancient woodland but these were more out in the open, between a mown area and a patch of gorse.
I tried Rose's wildflower keys for woodland and for open areas, no joy. Thanks for any light you can shed.