Clearing the sand has allowed me to see that damp is tracking across to the oversite when it rained last night.
I suspect that the external ground level is too high. Only a single course below the dpc visible. We've not been here long but I intend to remove the decking and the slabs. Remove ground to reveal at least 3 courses and back fill with 20mm gravel.
Anyone had a similar issue? How did you resolve it?