laying a slab on top of ground thats been compressed by a digger demolotioning a house

I would still Wacker the Blocks / Hard Core down,until fully crushed and wacked down,then infill further where needed, simply knock some pegs in to the level required as a guide and wack down to the top of the pegs. Once concreted I would suggest a expansion joint and / or induce a crack,by slitting the Slab with a Stihl Saw If iam understanding your question right? Usually ,single skin Blockwork ,obviously dependant on height would be fine on top of a slab you are suggesting. Also drainage will be important,with regards what your current drains capabilitys maybe and as to what you can add to with your new building,ie Rain water / Surface water, Levels etc of the current drains and as said above what can be added to / or not. Is further a Drainage needed ? Further excavation work / Removal of Spoil etc? All needs taking into account. Hope somewhere in all that you have a clearer picture