There are several patches of Cardamine quinquefolia in flower at the South London Botanical Institute. Hailing from east Europe I'm surprised it isn't grown more in UK gardens; seems to be vigorous, shade tolerant and flowers early in the year.

 Hopefully I've identified it correctly. It bears a resemblance to C. bulbifera and Cardamine is a large genus. I'm co-leading an event at the SLBI this Saturday to update the inventory of plants in the garden. Fortunately one or both of the SLBI's gardeners will be there too and the renowned botanist Roy Vickery. I'll rely on them for any tricky plant IDs! The event has been well subscribed I believe and should be a nice day for both experts and beginners.

 Postscript 19/03/22  Wiser heads than mine weren't entirely sure which Cardamine this is. So for now calling it C. quinquefolia is a definite maybe, it may be definite.