The builder has spent the day kangoing holes into walls and floors, looking for the source of the damp that is present in the party wall, and that destroyed the parquet floor that was underneath the engineered floor in the porch.
Checking the gypsum and wall 
Hall tiles up, needs levelling 
Yep, just levelling 
By stairs kango hole 
By-stairs with reference 
Leaking pipe under the radiator 
Pencil marks are the spread when kango’d 
Radiator kango hole 
Front porch pare back to check 
Close-up of the porch wall 
The layers on the party wall 
It’s damp in there, and rusty 
Hole in context in the porch 
A suspected culprit
Tomorrow’s project for me will be to buy a hose, hook it up to the tap outside, and spray that wall with water for a few hours to see if anything shows inside the property. I might need a lump hammer and a cold chisel too, to take off a higher band of plaster.