We are a firm of builders that sub contract the plastering work. The only time a joiner would be needed were if you were dividing a room and he would do the wooden framework on which to fix the plaster board. If its just to even up a wall then wooden 'lats' are screwed to the existing wall and board placed on top which is usually done by the plasterer. You do not NEED plasterboard if you are just having a skim to neaten things up. I suggest you get some more quotes.
What are the walls? -stone or brick -you can dot and dab modern brick but it does not work so well with stone and you may need lats and insulation. All of this could be done by the plasterer
I think yer man is a bit of a div. joiners don't usually fix plasterboard. They don't usually fix stud or framework. That's for a carpenter. Joiners make doors/windows/storey frames/staircases etc etc.
On sites, plasterboard is often fixed by "tackers." In your home, plasterboard is fixed by anyone who is competent. Carpenters certainly, but any decent plasterer will also do it willingly.
We do need to know the type of surface you want to fix the board to.