One of the factors that drove me this way is that the house is significantly colder in high winds - so there is air leakage somewhere and we have sanded floor boards on the ground floor. Windows and doors are well draught-stripped and there is no discernible air flow around any of them. Fitted carpet might have been one solution but it looks wrong in these houses. The void below is very dry as there is very good through ventilation - the void area floor is garden level at the back. But if the under floor insulation doesn't help it is hard to see what might and it was free (70 sq metres - I've not priced that).
I am very familiar with Edinburgh floors, having had two Georgian flats in the New Town before this modern, 1897, house. The deafening (not deadening) is intended mainly for sound insulation by solid mass. It has an air gap on top of the ash and plaster which is ventilated and the air moves well through that. In the Georgian flats there would have been little need for heat insulation - owners were wealthy, coal was cheap and when we did light coal or log fires the heat was astonishing. The sound insulation effect was very good - not possible to hear people in flats above or below.
Likewise in this house - the servants' quarters are quite large: bedroom, parlour, scullery, pantry and lavatory all separate from the rest of the house: heating costs irrelevant - we had 7 fireplaces. Now the house is occupied by poverty-stricken pensioners, life is different. I do plan to buy one of those IR thermometers.