First thing, WEAR A MASK WHILE SCRAPING.
I'd give that whole ceiling a good going over with a scraper first, removing ALL areas that look dodgy, loose,, flakey etc.. The tape that runs across the ceiling should be checked too, to make sure it's secure, and also check all other tapes/joints too, re-tape if necessary.
After that, I'd give the whole ceiling a coat of PVA to seal it all in,, LET IT DRY.
Now for the plastering. This is what I would do.,
I'd use a good coat of Bonding plaster over the whole ceiling first, flatten as you go, and then let it set. After it has set, (the same day), put on 2 coats of Multi Finish. You'll get a far better finish that way.
Before you put on the Bonding Coat, don't forget to PVA the ceiling AGAIN,, as you go, and apply the Bonding Coat onto the PVA while it's still WET/TACKY.
NEVER plaster onto dry PVA.
Another little tip,, I ALWAYS put a red food dye into the PVA , and give it a good mix in, it turns it pink, and that way, you can always see where you have been, especially onto a light surface.
The PVA should be thinned down a little with water too, according to instructions on the container.
The main thing though is prep.
A good plasterer would do that ceiling no problem, but if you think you can do it yourself, good luck to you mate.