Hi Paddy,
We use riccia stone pushed side on along the divide line although we imagine that thin ,flat pieces of slate will do much the same job in forming a barrier.
Once plant growth gets going any visible edges of stone are soon completely hidden whilst forming an effective dam between the sand and the Aquasoil.
The scape is looking good and we hope to get all your plants out to you tomorrow on a 24hr delivery.
All the very best,
Jim, Mark, Graeme and Ewan.