If the area is sunny, you could try lavender bushes, but choose the variety carefully, checking the height and spread of each -some are much taller than others. The trouble is, that three feet of border is likely to be very dry, nutrient poor and full of root matter from the hedge,and you may find that whatever you plant in it does not do well. Lavender does tolerate dry, free draining, nutrient poor soil conditions, and doesn't make a huge root system, but they will need clipping over twice a year after flowering, and if they survive, you will likely need to replace them after 5-7 years because they get leggy and woody. Keep well watered after planting up until late autumn/winter.

Another piece of advice - given the presence of the trees in that area, I wouldn't use a membrane nor slate chippings. It would be better for any plants you put in there if you add some composted manure or good garden compost to the soil prior to planting and use the same material as a mulch over the top, but if you don't want to do that, then plant into the ground and use the composted manure just as a mulch on top, preferably 2 inches deep minimum, adding it yearly. This will break down and improve the soil and increase moisture levels, whereas a membrane with chippings will do nothing at all for the soil or the plants. If you get the spacing right with the lavenders, they will form the equivalent of a low hedge and cover most of the soil over time anyway.

Thanks for that, lavender was one of the plants I was thinking of to form a small hedge under the lellandii.
I appreciate the advice against using slate chippings & a membrane base