You have a great idea for a low retaining wall using flags on edge. But do you have any ideas for draining water through if there is solid concrete haunching? I'm sure this wall would be a far cheaper option than a mortared random stone wall retainer that I have been thinking about! Retaining edges of planting beds vary between 150 and 300mm and indian sandstone flags that are being used in the garden anyway could be used here. Your website is the bees knees!
Dead simple - embed short lengths of narrow diameter pipe into the haunching so that it coincides with the joints between each flag. You could use 32mm or 40mm waste pipe, or even summat like 18mm overflow pipe - as long is it extends beyond the haunching and into some granular material, it'll be fine.
