Ok, first things first: yes we will be getting a plumber (?) to look at this, but I know how quick the great Mumsnet is at resolving this sort of mystery. So....
We definitely have damp in the understairs cupboard, inside a fitted cupboard sort of situation. It smells (not of rotting, but like a very intense "old book" sort of cellar smell), the paint is coming off the walls (it's sort of chalky textured and falls off when you poke it), the woodchip laminate style floor is kind of "popping" upwards, and the humidity doodah says 74%. It's an exterior wall. Outside there does seem to be a corresponding "hump" where the bricks are darker and I'm assuming are damp.
However. There are no water pipes that I'm aware of near there (the one running outside is gas going from the meter to the boiler). Although it does seem to have condensation or rain dips underneath it, I don't think that would be enough to make the wall damp and surely it would be all the way along. The first-floor bathroom is at the other end of house (at the back) whereas this is right at the front. So at the moment I'm stumped. Any ideas?
Photos attached of:
- the Hump of Darkness outside
- the wall above the hump of darkness outside
- under the stairs with damp wall, also showing the popping floor
Our current immediate plan involves removing the fitted cupboards, then try to lift the flooring and see what's underneath. Also locate a plumber who would be able to do something or investigate more thoroughly.