We've decided to finish the Lounge floor in flagstones, and since we found that there is nothing except a mixture of sand and mud underneath the inch of concrete in that room, we've decided to dig it down a little and add a layer of insulation.
There was no avoiding the manual labour of digging, raking and harrowing out a cubic metre of old crud - which is now forming a nice spoil-pile behind the compost bin.
It was hard work, as the sand mix contained many stones that stopped a shovel going in smoothly. Eventually, however, we got down to around about the desired level.
I fashioned a basic level gauge so I could use the hearth stone as a datum level and try to get the rest of the room vaguely level compared to it. I set the depth of the upright at 150mm, which gave me 30mm for the flagstone, 20mm for the lime screed which it will be bedded on to, and 100mm for an expanded glass insulation material called TechnoPor, which is non-porous and thus will not wick moisture up from the earth to the flagstones. It will, however, allow natural evaporation as the temperature of the room gradually rises.
A good stamping and levelling with the side of a scaffolding board got the room to a rough sort of level - good enough to have 1.5cu M of TechnoPor poured on top of a geotextile membrane anyway.
The advantage of TechnoPor is that, despite being glass, it can be compacted using a whacker-plate so that it forms a solid, strong, structural base for the lime screed.
You can see what's going on depth-wise here, and the angle we've left underneath the hearth stone. We've left the same angle at the edges of the room, since the foundations on the house are very shallow.
And here you can see the geotextile membrane in place, waiting for the TechnoPor to arrive.