I am trying to fix a plaster / lath ceiling. Near one corner, a section
approximately 2' square has fallen out, and the rest of that corner is
sagging visibly. It looks like the plaster has become detached from the
lath there, but the lath and joists seem solid. I have a few questions
about effecting a repair.

* Is it practical to only repair the damaged area? One option I was
considering was removing the sagging plaster and screwing plasterboard
into the joists below the sagging and the hole. However, it seems a
reasonable assumption that if one part of the ceiling is damaged, the
rest will be soon.

* Is it possible to screw entirely new plasterboard over the sagging
plaster, and the hole, and the existing ceiling? This seems perfectly
possible from reading
http://www.diyfaq.org.uk/miscellaneous.html#repairing and similar, but
the problem is that the existing ceiling has stippled plaster.

* Regardless of which option above is best, is it cost-effective to do
the plasterboarding myself and then pay a plasterer to do the skimming,
or should I just get the plasterer to do the whole job if I'm not
planning on skimming it myself?

Thanks for any advice,

Dave