David, thanks for that. Concrete gravel posts are a neat idea (and would
look better than blocks too, as the visible upper part of the rear wall) but
how would you suggest I install them? Or would simply resting them against
the fence posts one above the other be enough, letting the infill hold them
in place? Then the question of the outward pressure would arise, wouldn't
it?

The posts are well sunk into the ground and concreted in place. I'm
confident that they would hold against the pressure from say, an eighteen
inch bed, but I'm not sure about twice that height. Or is the bulk of the
pressure downwards rather than sideways?

I hadn't thought of sleepers, so thanks for the idea.

I've known treated OSB have a decent life outdoors but admittedly I've never
used it in a situation where one side of it was effectively buried in stone
and soil. Perhaps if I covered it with roofing felt?