Almost vertical banking!

I would try plants that can trail downwards too. Plant them at the top of the slope. Try ivy or Cotoneaster dammeri, these will both cope well in the shade. If you grow them downwards there won't be any chance of them flopping away from the slope especially if there is no means of support. Or try pegging bean netting over the soil if you have to plant at the bottom of the slope. Both these plants will root along their stems for extra support and will grow on clay.

I would not even think of planting grass which will need mowing as it will be almost impossible to do safely. Vinca major 'Variegata', Ajuga reptans or Pachysandra terminalis 'Variegata' are all reasonably small plants which should not be affected by wind but will give you some flowers. The cotoneaster dammeri would also be an excellent choice. Juniper horizontalis Glauca is a lovely low growing evergreen which will eventually spread out about 9' or more. It is hardy and tolerates poor growing conditions. It can be pruned to keep it to its alloted spot.
Before you start any permanent planting please ensure your slope is weed free because it will be a nightmare to get rid of weeds afterwards. Your low maintenance area can become a source of irritation. As well as putting down netting as suggested by Volunteer think of putting down a membrane and plant through that. Put a mulch on top of the membrane so that it retains moisture instead of cracking in the summer heat. Whatever you plant should be well watered for the first few months to ensure it can put down roots and survive. You say there are rocks, are they rockery size rocks (if you bury them up to 2/3rds of their bulk would they be seen). You do not say how long the area is. Could you cut paths in to it shoring up the cut out edges above the paths using the rocks on a slope. Click on S in the A-Z at the foot of the page then on 'Sloping garden ideas'. Plenty of food for thought there to inspire you.

I have exactly the same area and clay soil in my garden.

I have planted some different types of Juniper and a low lying pine call Pinus Mungo in the parts of the slope that gets semi-shade. In the full shade part I planted Acuba Japonica, which is a lovely type of Laurel, a bamboo called Fargesia Murieliae, which is hardy and Fatsia Japonica, an exotic looking, shade loving plant. I have just planted a Forsythia and a Japanese Flowering plant whose name I forget that I bought for 2 for £5 in Tesco. Both will do well in the shade and will provide yellow and pink colour.

My plan in the future is to add some ornamental shade loving grasses and some hostas - so the area will have different green colours, shapes and shadows.

I have tried to leave the stones on the slope, just in case heavy rain causes problems, but it is difficult not to pick up a few everytime I go down there.