I'm not a great believer of a sand base for flagstones. I have a
walkway/path of them, and the sand tended to just wash away, and the
flagstones needed to be redone. I had the best luck of laying them on
a bed of gypsum and fine gravel. I filled the spaces with pea gravel.
They are much easier to maintain now.

Myrl Jeffcoat
http://www.myrljeffcoat.com