I think that the layer of sharp sand will give far too much drainage and starve the turf of water.

Sharp sand is ideal for adding openess, air and making soil freer draining say in heavy areas. Depending on the surrounding levels, remove most of the sand and dig in the remainder into to top soil beneath. If you need to add top soil as maybe they have removed original cover down to clay, then loosen the subsoil and mix with the remaining sand then add the top soil.

Think of adding one ten litre bucket of sand to a barrow of soil to get an idea of the ratio required.

Just a thought, youre certain that its sharp sand and not fine sticky builders sand. Sharp sand is nice and gritty and doesnt stick together when compressed in your hand. Remove lumps of dry morter if the flags have been laid over piles of cement.

When turf is laid, mix sand with fine sifted soil and fill in the joints. Dont trim the turf back to your line until your sure the truf edges have 'taken' or you may get a bit of die back which may disturb your boundary line.

OP