They actually talked about this earlier in this series of Gardener's World. You can still find it on iPlayer here:

http://www.bbc.co.uk/programmes/b01dy7nz

The basic idea (as I remember it!) was to plant things that spread and self-seed at the top, and over time they will colonise further down the bank: depending on whether the bank runs up from your garden or down from your garden, that might make it easier or harder I guess! But either way limits the number of plants you need.

DeB