nonymouse wrote:
Thanks for your suggestions, but...

Render and skim.

...do I need to take off all of the plaster first then, or only the
loose stuff?


If it's damp (and it sounds to me like you've had water running on it for it
to be damp, unless the bathroom is downstairs in which case it could be
rising damp, or possibly an outside source - leacky gutter, downspout etc)
then it will probably all be loose (another IF here - if it's oldish plaster
that is, if it's less than say 30 years old plaster than you may get away
with just taking the loose off and skimming over it all)

Is it a 'wet' wall? - is there a shower or bath against it? - if
there is then you don't have many options...personally, if it were
my house I would hack the lot off, allow it to dry (the water has
probably been getting behind it for years), PVA, two coats of
render, then tile over

One part of the wall will be a wet wall which needs tiling and I don't
fancy tiling over manky plaster. The other part of the wall joined via
an L shape will just be tiled

Anything that you are tiling will need a solid base, the general rule of
thumb is: if you think it /might/ be loose, even just a little bit, whip it
off, either down to bare brick or just the top coat of plaster, apply 2
coats of PVA and replaster.