what i am now proposing is to make up my plant pot soil with 50% of
this garden soil plus 25% garden centre compost and also 25% sand.
Does this sound like a reasonable proportion ? and if so what kind
of sand should i use please ( sharp sand or smooth sand ?)
It should be OK, as it isn't usually critical; but I'd up the
"compost" proportion a bit, as you want good water absorbency in
pots. You may find that in use your topsoil makes the mixture turn to
concrete when it's dry: worth trying some out in pots without plants
until you get a mix which stays crumbly when dry and doesn't stick
together like dough when wet.
Use gardener's sharp sand, not builder's: biggest bits an eighth of
an inch across.
But your mixture will still get exhausted in pots, and you'll still
need to feed in a year or so. I generally use the stuff out of
growbags -- sometimes even used ones.