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.