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Doggit
If the house was built in the 30's then I don't think there's an issue with the foundations, and most of london is built on clay, and it expands when wet, and shrinks back when dry. That's the reason that most houses built on clay, have a raft design for the foundations. If there is no evidence of any cracks in the walls, then everything is fine. The DPC will be about 6" to a foot above the ground level, and can be seen by a blackish line, or a thicker mortar bed. This stops timber etc getting wet, and keeps the house dry.
But in you case, if I remember the start of this thread, you've got issues with damp under the floorboards, so some concrete with a waterproofer added and poured under the floorboards to provide a seal, might work best; it wouldn't stick to the clay, but set above it, and keep the water out.
But in you case, if I remember the start of this thread, you've got issues with damp under the floorboards, so some concrete with a waterproofer added and poured under the floorboards to provide a seal, might work best; it wouldn't stick to the clay, but set above it, and keep the water out.