Honeysuckle would grow over the wall and there are lots to choose from, including early and late flowering varieties to keep the show going. But your choice of plants might be influenced by which direction the 'down' side faces. If it is not on the sunny side with a southerly aspect, any trailing plant might not spread on that side.

For poor soil try - Phlox subulatas such as 'Emerald Cushion Blue' and 'Candy stripe'. Aubriaeta 'Cascade' Red and Blue. Snow-In-Summer (Cerastium Tomentosum). Dianthus deltoides 'Flashing light'. Dryas octopetala. I grow all of these and all grow well and will flow over your wall adding interest and colour.

Aubretia will trail for a short distance over the wall and flower for most of the year. Our own native purple saxifrage, Saxifraga oppositifolia will do similar. The trailing ivy leafed toadflax can be a bit invasive but will flower in shadier situations.
Do you have any cavities in the wall or can you drill some holes through to the soil behind? In that case you could grow some small alpines along the face of the wall, they would love it.
Then site some containers at the base of the wall with taller growing plants going up to meet the ones coming down. The whole feature could be very colourful.

Fuchsia Lena will hang over a wall for about a foot or more and is very floriferous and really beautiful. If your winters are harsh it may not survive but is worth a try if you mulch it well. Ours came through minus 10 last winter.

creeping jenny would look nice but a good mixture would be best . nothing to agressive growing as you dont want your wall cracking . i wish i had such a wall im so enviuos . to me its crying out for a waterfall .

Yes NP, with such a nice long one there's room to try all the suggestions!

id like to somehow make pockets up the wall and plant in them but i cant think of a subtle way of doing it . it would be nice to have a photo royrob .