Got a damp meter and have been playing with it. The wall with the tidemark gives 17.7 above the skirting, 13.5 about a meter above, and 10.3 about 2 meters above. The adjacent wall gives 20.0 above the skirting, 4.0 about a meter up, and 9.0 about 2 meters above.
The floor reads between 15.0 and 20.0 along the wall with the tide mark. I can't get the carpet back any further to check further out, but once partner comes home we can do that if need be.