You don't say whether you want shrubs or herbacious perennials. It depends on how tall you want the plants to be. At the risk of pretending that I know what I am talking about, this is a time when invasive plants with deep roots and underground runners have their uses. (Japanese knotweed was introduced by the railway authorities to stabilise the banks of railway cuttings). You could use an erosion control sheet or even some plastic to stabilise the bank and then plant through that, too, and perhaps take it away when the plants have matured somewhat. If the site is sunny, you might find that things like lavender, choisya and cranesbills (perennial geraniums) worked well. Japanese anemones spread well, and are tough. Vincas provide strong ground cover. Ornamental grasses like carex and festuca might be good. The RHS website has a garden planner where you can feed in information about your needs and get a list of plants. Just type rhs,org,uk/plant selector into a search engine. Hope some of these ramblings help.