Sand is actually better than the crap you often see on development sites! A naturally occurring clay subsoil is extremely vulnerable to compaction, and you see the resulting drainage problems commonly on new builds. I work as a landscape architect, and we are specifying a layer of grit sand beneath the topsoil on one of our trickier sites (not as deep as your sand though - about 300mm). The sand layer intersects with the drainage layer at the back of the retaining walls, which in turn drains out via weepholes.
"What is hateful to you, do not do to your neighbour".
I still could do with guidance on depth of soil for flower border with the odd fruit tree
30 cm is fine, after that plants/trees roots will happily go into the subsoil for anchorage etc
If you have a garden and a library, you have everything you need.”—Marcus Tullius Cicero East facing, top of a hill clay-loam, cultivated for centuries (7 years by me). Birmingham
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I still could do with guidance on depth of soil for flower border with the odd fruit tree
East facing, top of a hill clay-loam, cultivated for centuries (7 years by me). Birmingham