Home DIY

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 :)