I recently had a new ceiling light fitted which replaced a standard plastic ceiling rose. The new fitting is quite slim and the old fitting was hiding a larger than expected cutout in the plasterboard.
What would be the best way to fill in these gaps around the edge of the new fitting? My current skills in this area extend to Polyfilla but I don't think that will work in this situation, at least not without some type of reinforcement. The fitting can be accessed from above.
You could install a circular ceiling medallion over the hole and run the light fixture through the middle of it. This would avoid having to match the paint of the surrounding ceiling.
Or here's an even simpler, 10", white trim piece.
Like any other repair to sheetrock/plasterboard/drywall.
Turn power off (the breaker, not the switch) to the fixture and drop the trim out of your way. Either cut out a larger area and make a large patch, or add some wood strips behind and fit a small patch, then fill the joints (force joint compound into them with a small drywall knife), tape, and mud (apply joint compound to the surface with a drywall knife) - repeat until smooth, don't try to get it perfect the first go, sand off bumps, fill in hollow with the next coat, use harsh lighting from the side to assess the state of the repair, when fully smooth, prime and paint. When paint is done, replace the light fixture trim and restore power.
You can use a "Goof Ring" or a ceiling medallion to fix this problem. It seems that ceiling medallions are more for a decorative fix and seem more bulky and harder to install but goof rings are more discreet and a quicker install and fix the problem for much cheaper and easier in my opinion. Some pictures below from a product listing for acrylic goof rings:
