Slope about 1 in 30 down towards front elevation of house.

Top soil about 150mm. Limestone brash soil, high permeability.
 

Trench fill, topped by concrete blocks cavity wall (lean mix fill). Backfill with limestone brash spoil. Sand blinding 50mm, DPM, Concrete slab 100mm, insulation etc.

Do these foundations need to be waterproofed? Else will water simply run under the slab and swell or shrink whatever little clay remains in the limestone brash? (Moving the underslab drains)

 

No.  The foundations should extend down to where there is no risk of shrinkage or heave.  If the floor slab is susceptible you will need it to be suspended / beam and block.