nonymouse wrote:
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 :-)

If it's an exterior wall, I would take a closer look at the outside
brickwork, maybe it needs re-pointing, 25 year old plaster shouldn't
deteriorate so much without a fairly substansial amount of water penetration
over a prolonged period....humidity from the bathroom shouldn't affect it,
after all, the plaster in our kitchen is 60 years old and that has had pots,
pans and kettles boiled in it thousands of times