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.
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.
Now all that’s left to do is create the concrete dish on top. Once that's done I can carry on with completing the deck.