How do I bathroom / kitchen / extension? etc.

Yep. The builders put wooden battens across underneath the joists, popped about 10cm of insulated panels in there, and sealed up joints with expanding foam. The old floorboards - much to the horror of my architect who has tried to talk me out of it, no kidding, about ten times - will go down in a few weeks, get sanded and sealed. The new bit of building has a concrete floor and battens will go on there so that reclaimed floorboards can seamlessly (hopefully) run throughout. So much of the old floor was rotten and joists and all sorts of supports are being replaced. Makes you wonder how many Victorian houses in London are in need of the same treatment.