We always have nasturtiums in the shallow beds on top of the garden walls for a bit of colour around the farm, but I'm looking for a bit of a change. I did successfully manage to grow sweet peas in the same bit last year but didn't really have time for the maintainence of them so they ran away from me, and will I have even less time this year. So it needs to be easy, although I won't rule out trailing/creeping plants entirely. However, they also got eaten by the sheep DP decided to put on the bit of grass next to the garden and the chances of that happening again is extremely high so ideally nothing too expensive or poisonous to sheep.
Between 2 - 4 inches of soil in depth, mainly south facing and sheltered but a good 1300ft above sea level and in the far far north so temps don't get that high and we tend to be a few weeks behind the rest of the uk - e.g the daffodils are only just starting to peek out now.
I might try a mix of the suggestions and see what works, there's about 15 metres of wall to cover so plenty of space!
I should have remembered really I was ideally trying to avoid aubretia and campanula as I already have lots and lots of it growing everywhere else out of all the other walls (someone in the farm's past decided to put planted walls everywhere and as a result they are almost seen as weeds here), BUT it has made me rethink that it perhaps would be worth putting some it as a bit of general filler for the years where I don't get round to doing something a bit different.