In article , "Tim Smith"
says...
Firstly, we have a small (2.5' by 2.5') ceiling at approximately 45deg over
the stairs between the horizontal downstairs ceiling and the vertical wall
of the stairwell. The plaster on this bit of ceiling was completely shot
falling off. Underneath are the wooden slats attached to three joists.

Since Im not too bad at plastering now (did a course near Bedford and plenty
of practice) I tried to replaster this with the normal sand/cement mix
undercoat. Of course I had great difficulty in getting much of this to stay
in place.

You should have used bonding coat, not cement. It's really not too
hard to plaster wooden lath ceilings - I just scrape on a layer of
bonding to fill the gaps, then level it off with another coat
straight away. If you need to build up a thick undercoat it might be
better to apply a thinner layer and let it firm up before adding
another layer - I've had sags that I had to scrape back before I
could apply the topcoat, but that might have just been an overly wet
mix.