scraper is spot on with advice, I would also advise you not to put in decking, besides being high on maintenance its dangerous, inadequate light,heat on the north facing garden causes a slippery moss layer on the deck boards which can cause you to slip and fall.
Sandstone should be layed on plastering sand/cement mix, 6 to 1 is the spec, (on a whacked hardcore base) as gravel wont bond properly with slab.
Rompox or pavefast are by far the beat method of sealing the top of the slabs, its basically a sand you brush between the joints that turns into concrete in a couple of days wihout staining the slab