Do we dig out our old quarry tiled floor and replace with concrete/insulation/dpm?

If the quarry tile is stable there is no reason to remove it and besides , messing with original design is always going to cause some unexpected and costly remedial treatments . Make a waterproof slurry out of SBR and neat cement mixed in a ratio of 1:1 by volume . Apply it to the whole floor area. Screed or cover any way you like after that .

The old tiles and ash sand will be wet and damp you do need to remove them because the tile is above the dpc already in the house the dpc in the old house is likely tar or slate but no matter water proofing over the tiles will stop the damp coming up through the floor , but the damp will rise up the walls of the house . To do it rite you dig the floor out enough to allow for the new floor build 1 / a small bit off blinding stone 2/ dpc sheet over the top of that turned up the walls rite up past floor level 3/ sheet insulation from 100/200mm deep which every suits yous best 4 / dpc cover sheet over the insulation 5 /50mm insulation strip round all walls 6/ 100mm max off screed over the top When you have the floor dug out it would be wise to inject dpc into the walls along the mortar joint around 100mm below your new floor level cost ways if the room is stripped out the dpc injection is cheap and if your old dpc is not broke down already it will be long before that floor breaks down Thanks

That but lay with DPM