Many thanks - it is an exterior wall with no guttering above. The
plaster is around 25 years old as this part of the house was renovated
in 1981 (I think) - and hasn't been re-decorated since!

There's no reason (other than it being an exterior wall) for it too be
damp as it has not been in constant (or even semi-constant) contact
with water; I can only think the damp has stemmed from the humidity in
the bathroom

Bear :-)