The damp in the kitchen walls is off the scale of the damp meter I am using. One of the many causes was that the flaunching in the drain outside the kitchen had failed due to water falling through misaligned pipes wearing the mortar away.
The sink drains via a tortuous route into the grey outlet at the top left of the photo, then the water falls in the gap (just to the right of the spade) and back through the kitchen wall just underneath the sink.
The builder next door suggested a strong mix of 1:1 cement and sharp sand, and tapping the cement to bring the ‘fats’ to the surface to make the cement smoother and stronger. He gave me half a bag of cement and some sharp sand. It occurred to me later that flaunching around drain pipes without much access wasn’t necessarily the easiest job for my first time working with cement (I’ve never owned a house before). But it turned out OK. It was splashed with water shortly after it was done which is why it is a bit blotchy.
I still need to re-align the drain pipes. The modification to the kitchen sink drain will be temporary. Kae has decided to move the kitchen into the barn.