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

Worrying thought...the house was built in 1901!