Campanula poscharskyana makes a good cover for ugly surfaces and survives under difficult conditions.

I've just bought a packet of 6 types of herb seeds in one pack (all in little separate packs)from 99p store. These included Thyme. I sowed them last week and the Thyme has germinated already! Try these and then it will be quite easy to poke the little seedlings into the gaps. Or buy some pots of different sorts (variegated etc) and snip off little cuttings and insert these into the soil in the wall. Maybe cover with plastic initially to keep them moist?

Hi, Iago---Why don't you try house leeks Several different types and colours) as they love dry places. Also aubretia will spread nicely and doesn't need much soil or moisture. Lewisias love to grow in a vertical wall as they need lots of drainage. Whatever you plant though, you must put lots of grit in the soil as all these rockery-type plants need good drainage. Good luck, Annie.
P.S. You could also grow something like lavender or box in front of the wall to hide the ugliness at the base.