jay
my swing mooring at shute harbour in 12 meters of water consisted of 1/2" chain led through the bow roller with a small loop shackled into the end. i passed a 1" nylon rope bridle through the loop. the eye ends of the bridle were made fast to 10" bronze deck cleats about a meter and a half back from the bow. the chain was fastened near the middle of the bridle about a meter forward of the cleats. the top 2 meters of chain was fed through a tough blue rubber fire hose for chafe protection. the chain was fastened to a concrete block of unknown size on the bottom. there was also a large mooring ball and a small pickup ball attached to the 1/2" chain. i can draw a detail and send it to you if you want.
queensland required an annual survey of the mooring, but new south wales does not. the certified surveyor would dive the chain and hammer every link of the chain and replace some of it and then add a new 5 kg hunk of zinc.
when i left the boat for more than a week or so, i backed up the system by tying another long bridle from the aft cleats forward and through the loop in the chain.
but...., a work of caution. on one occasion when over 30 boats were lost to a strong northerly at airlie beach. a W32, "soya" drug its mooring block a mile or so and ended up on the rocks. i dont know how, but you should probably try to determine what size block will hold on your sandy bottom.
james