I think that the layer of sharp sand will give far too much drainage an
starve the turf of water.
Sharp sand is ideal for adding openess, air and making soil free
draining say in heavy areas. Depending on the surrounding levels
remove most of the sand and dig in the remainder into to top soi
beneath. If you need to add top soil as maybe they have remove
original cover down to clay, then loosen the subsoil and mix with th
remaining sand then add the top soil.
Think of adding one ten litre bucket of sand to a barrow of soil to ge
an idea of the ratio required.
Just a thought, youre certain that its sharp sand and not fine stick
builders sand. Sharp sand is nice and gritty and doesnt stick togethe
when compressed in your hand. Remove lumps of dry morter if the flag
have been laid over piles of cement.
When turf is laid, mix sand with fine sifted soil and fill in th
joints. Dont trim the turf back to your line until your sure the tru
edges have 'taken' or you may get a bit of die back which may distur
your boundary line.