Hello
I'm a real fan of the bugle plant, for it's quick growing ground cover & pretty blue flowers in Spring. It will spread, quickly but gives a ground cover all year long.
http://www.telegraph.co.uk/gardening/6043863/How-to-grow-bugle.html
It might get established in the wetter parts of your bank & then spread else where. I've grown it a pot before and I think it can stand peroids of drought. It's hardy, grows in partial shade/shade,needs no maintence, easy to dig up & very easy to propogate. It's my type of plant.
Also what about the yellow primose?
http://en.wikipedia.org/wiki/Primula_vulgaris
They seem to grow everywhere, spread like mad & look lovely in spring, just let them get on with it
Spring bulbs in my experience don't need much look after, just plant loads
I agree with a previous poster,lasts week GW show had a piece on planting banks.