The wall is downstairs in the hallway. Immediately above it is the landing. I have lifted the floorboards up and couldn't see any pipes running near it.
I'm trying to find the source of a leak. There's a damp patch on an internal wall about 1m above floor level. I don't know if it's a leak from a pipe or ingress of water from outside. I've taken two readings of the heating system pressure one week apart. The Central Heating was not switched on...