So, after figuring out where my posts for the decking would go I dug a trench between them which continued under the sleepout in a downhill direction.
There is always a lot of water seeping through the soil after it rains so I opted for nova-coil piping which has lots of little holes in it which will act as a kind of sieve so all the water can drain out of the soil and escape down the pipe. The free draining volcanic scoria will also help the water to drain away. The aim was to create a kind of soak hole outside the perimeter of the sleepout which will collect any of the water seeping through the soil. Then, by means of a vertical section of drain connecting to the nova-coil the plan was to finish off with a layer of concrete shaped like a dish to collect any surface water and feeding it down the same pipe. 
This required a bit of support by means of a small retaining wall which would essentially act as a dam holding everything in place.
I also dug a trench under the sleepout using more nova-coil pipe and back filling with scoria. I made sure there was a reasonable gradient on the trench so no water would sit and pool anywhere but instead would drain away quickly. Hopefully this will reduce the amount of moisture sitting around under the sleepout and this will contribute to keeping the sleepout warmer and dryer than before.
Finally, I created the concrete dish on top. Now I can carry on with completing the deck.
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