This.
It's not a difficult job at all and your only really paying for the labour. It literally involves smashing down the rest of the ceiling and laths and getting it neat and tidy for the builder (if your not confident at boarding). All he'll do then is screw up new plaster boards and then (if he plasters as well) skim the boards. All you have to do then is repaint. It's a day or two job if you knew what you were doing and even less if you didn't need to wait for the plaster to dry before you can paint.
This is assuming you have no damp issues ^^
