Well, it turns out it’s a good thing I asked the builder to strip the kitchen interior wall (shared with the living room); that rotted bit of wood that we thought only held up the steps above it also holds up the RSJ that braces the bedroom floor, the balcony joists, and the roof at the front of the house.
The damp that showed up in the kitchen/breakfast room corner appears to be coming in along an old pipe line. We can seal it up, but the problem goes deeper and further. The concrete slab under the tarmacadam runs towards the house, and the tarmac itself is not well compacted; these facts lead to water pooling in the corner by the pipe. Then the haunching on the other side of the wall (neighbour’s property) is crap and needs to be replaced.
On the plus side, the plumbers are making progress with the moving of the boiler and cylinder. They’re going to have to go wider than initially planned due to how some existing pipework comes through the wall, but I’d already figured they’d need to do that, and was planning for it.