We were talking specifically about houses with suspended floors, and the majority of houses that got CWI were older, say between 1900 - 1940, the vast majority of these had suspended floors.
5 days a week, 3 houses per day, 48 weeks a year, for ten years = 7,200 houses...all over the north of England (and Wales), from Huddersfield in the East, Telford in the south, Wrexham in the west and Penrith in the north...a fairly large area by any standards....almost all had a DPC, mostly bitumen, but occasionally slate.