i would get a small bag of bonding plaster. apply in two or three stages allowing each to harden (don't have to wait until it has completely dried, just until hard enough to apply another layer).
with the final application use a plasterer's trowel to make it flush with the wall. once set you can use either finishing plaster or even polyfilla and a light sanding to get a nice skimmed finish. not a difficult job at all, just make sure that however you do it that there are no live cables still lingering. wet plaster (or wet anything) and electrics don't mix.