I have a similar space and I grow hellebores, ferns, cyclamens, Japanese anemones ( these need staking but they flower well), erythroniums, geranium phaeum 'Lily Lovage', geranium endressii, pulmonaria, sarcococcus and variegated euonymus and at the back a clematis montana that I keep well pruned after flowering so I get good leaves through the summer as well as low flowers in the spring.