The wall is built on a shallow bed of ballast, the same material used to infill behind. I should have ordered another 5 bags of that!! The trackbed is then sharp sand that has been levelled before messing around trying to get the track down in the right place! Now it's down it's not coming back up!
Tomorrow we'll nip out for an extra couple of bags of ballast, sharp sand and also try and source the track ballast, horticultural grit or the like I expect.
I'll sign off there... Another update tomorrow!