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This is really incredibly helpful, thank you. I've currently got tenants in my flat and I really don't want to impose my very slow, swear-ridden and ultimately unsuccessful bodging on them! Looks like I may need to try an wrap a few more jobs into a package and see if I can find a decent handyman? I've got plasterers coming to quote but from your message it sounds like they're unlikely to be particularly interested...
Cheers
owl-
I hope it's sheds some light on it from a decorators point of view. Plasterers might view it differently. If you get 2 people in for an afternoon they can patch that pretty quickly and skim it. They probably won't want to come back and paint it but you should leave it for a few months to dry if it's plaster and you can tell the tenants that while you work out how to get it painted. A painter would be happier to come in a few months later and paint the entire ceiling. Maybe that's the best approach.
Airhead
It's not that complicated to fix but you're right, trades don't want to take it on on its own. It's easier if you are decorating the whole room but decorators might shy away from it. Plasterers don't really want to get involved for something that small.
I would do it by cutting the plasterboard back to the nearest joists and cut it halfway down the joist. Then add a piece of wood between the joists. Measure and cut the square from fresh plasterboard. Screw plasterboard to joists and noggins. Plasterboard should be roughly the same depth as the existing ceiling. Tape the joints with joint scrim. Use a filler like easy fill or even joint filler premixed if you want to be lazy. Lay that on fairly rough and let it dry, fill again when dry and leave to dry. Rub over with a sponge instead of sanding, if you're filling is good enough that will be all it takes, otherwise you might have to sand it. Paint with Gardz, then 2 coats of ceiling paint. You might need to paint the whole ceiling.
If you do that as part of a whole decorating job it's a few hours work, maybe 4 depending on the state of the joists. Problem is a decorator often doesn't carry the wood to put the noggin between the joists or even plasterboard, that can add an hour while they get hold of the materials. If you carry all the materials then you are only using bits at a time, my joint tape has lasted 4 years. Filler I always have. Gardz the same.
If you have to go out and purchase all the bits together it will cost you about £50 and you'll have a ton of material left over. So you've got a job that only takes 4 hours but would need at least 2 visits and even then you'd be standing around a lot waiting for stuff to dry, unless you throw in quite a bit more work and have someone around for 3 or 4 days so £500-£800 or so.
TLDR
It's probably going to add £80-£100 to a decorating job and that's pretty much the only way you'll get a decorator to do it. See my workings above :)