Minimans - many thanks for that tutorial - much as I expected, apart from the air to load the sand hopper coming from the locomotive. It goes to explain why there may not need to be a release mechanism below the top hopper bin, as once the sand is blown up there the loco is already underneath ready to receive the dry sand. I was worrying about sand left in the top container getting damp again, or how a sort of "tap" of "wc-flush" mechanisn would work with powdered grains rather than a liquid.
Looks like I need to find a smaller container for the top of the sanding plant. Till then it looks like crews will have to continue with a ladder and covered bucket!