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?

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