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I had the same problem recently and used one of these and decent Polyfilla to patch a hole in our cellar ceiling:
https://www.ebay.co.uk/itm/Giraffe-Plasterboard-and-Drywall-Repair-Patch-15cm-x-15cm/323371878636?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649It's a functional cellar so the finish doesn't have to be perfect, but when we paint over it I reckon it will look decent actually.
cc: @hugo7
hmmm ok if I screw plasterboard into the place where a light switch or light fitting has been what would I fix it to to get it close to flush?
Fix batten behind the wall, fix plasterboard to the batten, tape and then fill?