Starting in shallow soil - need help/ideas

I have just moved into a house where the last owners created a 3 tiered wall out of breeze blocks. On each level, the holes in the blocks have been filled with compost. I am hoping to be able to plant either some flowers that fill the space well (covering the tops of the breeze blocks, and adding colour), or to have trailing/draping plants that will cascade down the wall and cover it that way.

Either way the holes in the blocks are quite small, so the plants would have to be shallow growing. There is almost always direct sunlight from spring through till autumn, and I live in wales, uk so it's quite a damp place (although I do have a tendency to forget to water things)

Any recommendations? I will post pics up tomorrow.

 

rockery plants are used to growing in limited and poor soil.

This time of year aubretia and alyssum. All year round coloured thyme.

take out the old compost and replace with a mix of JI compost, soil, grit and normal compost and a bit of bonemeal.